Add an optional ImageId SSM Parameter Path

[keithduncan]

This adds an optional String parameter to the elastic-ci stack parameter list. If given this is expected to be an SSM Parameter Path. This cannot be typed as an AWS::SSM::Parameter at the top level because that would make it mandatory. Instead, if given, a substack with a AWS::SSM::Parameter::Value<AWS::EC2::Image::Id> parameter is brought in whose output is the value we need.

Example error when using an AWS::SSM::Parameter::Value<AWS::EC2::Image::Id> parameter with a Default: "":

Error: Failed to create changeset for the stack: substack-laundry, An error occurred (ValidationError) when calling the CreateChangeSet operation: Parameter AmiParameterPath should either have input value or default value

This change removes the UseDefaultAMI condition and replaces it with HasImageId and HasImageIdParameter conditions. Precedence is: the plain ImageId parameter, the SSM parameter path, then the default AMI mapping.

The given TemplateURL I’ve used here won’t work because it’s file system relative and we don’t use aws cloudformation package to preprocess the template. We’ll either need to add something to the build system to make this work, or pre-deploy the new template somewhere we can reference with an S3 url. I don’t anticipate this substack template changing often and would lean towards separately packaging and deploying the ssm-ami.yml template to S3, and using a static semver'd URL to this template in the main aws-stack.yml file. This would avoid any potentially complex additions to the current build system.

[chloeruka]

Looks good to me! Thanks for setting this up.