Implement sensible default values for placeholders in Template

[dhutchison]

I got caught out today with the error IndexError: Fn::Select - List size is smaller than the Index given.

This was because my template was trying to add a "uniqifier" to some resource names based on the StackId (part of a Service Catalog product) using this:

  rBastionSSMIAMRole: 
    Type: AWS::IAM::Role
    Properties: 
      RoleName: !Sub 
          - '${pAccountShortName}-ssm-bastion-${uniqifier}-role'
          - # AWS::StackId has this format
            # arn:aws:cloudformation:us-west-2:123456789012:stack/teststack/51af3dc0-da77-11e4-872e-1234567db123
            # Trying to capture the last piece after the '-'
            # As stack name could contain "-"s, split on the "/"s first
            uniqifier: !Select [ 4, !Split [ "-",  !Select [ 2, !Split [ "/", !Ref AWS::StackId ] ] ] ]

By default StackId should have a non-blank value.

Workaround is to set this manually in test case

# Set a non-blank value for the StackId
Template.StackId = "arn:aws:cloudformation:us-west-2:123456789012:stack/teststack/51af3dc0-da77-11e4-872e-1234567db123"