Update MIG example to use Terraform v0.12 compatible modules

[aaron-lane]

[ivankorn]

terraform validate fails on that: output
#32

[ivankorn]

This issue is blocked by 0.12 support for following 2 modules:

• terraform-google-managed-instance-group
• terraform-google-lb-http

[morgante]

@ivankorn You can replace the MIG with this module: https://github.com/terraform-google-modules/terraform-google-vm/tree/master/modules/mig

[ivankorn]

Great! Thank you, @morgante !