Fix linter warnings and related makefile bug

[ivankorn]

1. LInters checks fail on master branch

[10:38][user@host:~/workspace/google/terraform-google-memorystore]$ make -s
Running shellcheck
Running flake8
./helpers/combine_docfiles.py:31:31: W605 invalid escape sequence '\['
./helpers/combine_docfiles.py:31:33: W605 invalid escape sequence '\^'
./helpers/combine_docfiles.py:31:35: W605 invalid escape sequence '\]'
./helpers/combine_docfiles.py:31:37: W605 invalid escape sequence '\:'
./helpers/combine_docfiles.py:31:39: W605 invalid escape sequence '\ '
./helpers/combine_docfiles.py:31:41: W605 invalid escape sequence '\('
./helpers/combine_docfiles.py:31:61: W605 invalid escape sequence '\)'
./helpers/combine_docfiles.py:31:72: W605 invalid escape sequence '\['
./helpers/combine_docfiles.py:31:74: W605 invalid escape sequence '\^'
./helpers/combine_docfiles.py:31:76: W605 invalid escape sequence '\]'
./helpers/combine_docfiles.py:31:78: W605 invalid escape sequence '\:'
./helpers/combine_docfiles.py:31:80: E501 line too long (107 > 79 characters)
./helpers/combine_docfiles.py:31:80: W605 invalid escape sequence '\ '
./helpers/combine_docfiles.py:31:82: W605 invalid escape sequence '\('
./helpers/combine_docfiles.py:31:100: W605 invalid escape sequence '\)'
./helpers/combine_docfiles.py:32:45: W605 invalid escape sequence '\d'
./helpers/combine_docfiles.py:32:47: W605 invalid escape sequence '\d'
./helpers/combine_docfiles.py:32:80: E501 line too long (99 > 79 characters)
./helpers/combine_docfiles.py:35:3: E111 indentation is not a multiple of four
./helpers/combine_docfiles.py:41:80: E501 line too long (80 > 79 characters)
./test/verify_boilerplate.py:56:15: N803 argument name 'ARGS' should be lowercase
./test/verify_boilerplate.py:184:28: N803 argument name 'ARGS' should be lowercase
Running go fmt and go vet
bash: line 0: source: filename argument required
source: usage: source filename [arguments]
make: *** [Makefile:49: check_terraform] Error 2

2. Makefile has a bug in a source call here

[ivankorn]

Linters check after migration to tf 0.12.
Python and Docker warnings to be addressed in scope of this issue.

[16:56][user@host:~/workspace/google/terraform-google-memorystore]$ make -s
Running shellcheck
Running flake8
./test/verify_boilerplate.py:56:15: N803 argument name 'ARGS' should be lowercase
./test/verify_boilerplate.py:184:28: N803 argument name 'ARGS' should be lowercase
Running go fmt and go vet
Running terraform fmt
terraform fmt -diff -check=true -write=false . 
terraform fmt -diff -check=true -write=false ./examples/basic 
terraform fmt -diff -check=true -write=false ./test/fixtures/minimal 
Running terraform validate
helpers/terraform_validate . 

Initializing provider plugins...

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.
Success! The configuration is valid.

helpers/terraform_validate ./examples/basic 
Initializing modules...
Downloading git::ssh://git@github.com/terraform-google-modules/terraform-google-memorystore for memorystore...
- memorystore in .terraform/modules/memorystore

Initializing provider plugins...
- Checking for available provider plugins...
- Downloading plugin for provider "google" (terraform-providers/google) 2.11.0...

The following providers do not have any version constraints in configuration,
so the latest version was installed.

To prevent automatic upgrades to new major versions that may contain breaking
changes, it is recommended to add version = "..." constraints to the
corresponding provider blocks in configuration, with the constraint strings
suggested below.

* provider.google: version = "~> 2.11"

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.
Success! The configuration is valid.

helpers/terraform_validate ./test/fixtures/minimal 
Initializing modules...

Initializing provider plugins...

Terraform has been successfully initialized!

You may now begin working with Terraform. Try running "terraform plan" to see
any changes that are required for your infrastructure. All Terraform commands
should now work.

If you ever set or change modules or backend configuration for Terraform,
rerun this command to reinitialize your working directory. If you forget, other
commands will detect it and remind you to do so if necessary.
Success! The configuration is valid.

Running hadolint on Dockerfiles
/dev/stdin:17 DL3006 Always tag the version of an image explicitly
/dev/stdin:23 DL3018 Pin versions in apk add. Instead of `apk add <package>` use `apk add <package>=<version>`
/dev/stdin:17 DL3018 Pin versions in apk add. Instead of `apk add <package>` use `apk add <package>=<version>`
/dev/stdin:35 DL3003 Use WORKDIR to switch to a directory
/dev/stdin:51 DL3018 Pin versions in apk add. Instead of `apk add <package>` use `apk add <package>=<version>`
/dev/stdin:75 DL3003 Use WORKDIR to switch to a directory
make: *** [Makefile:61: check_docker] Error 1
[16:57][user@host:~/workspace/google/terraform-google-memorystore]$