Fix Event Streams when functions have direct reference to custom roles

[piohhmy]

What did you implement:

Closes #3142

The AWS::Lambda::EventSourceMapping resource was being created with wrong the DependsOn attribute if custom roles were defined using a string to reference a logical role name elsewhere defined in the serverless.yml. The EventSourceMapping resource was previously only checking for roles defined via a direct ARN or the Fn::GetAtt syntax. It did not support referencing a role's name directly (i.e.. role: myDefaultRole as documented here)

The result of this is that the EventSourceMapping resource would set the DependsOn to the default IamRoleLambdaExecution role. If custom roles were used for all functions then the IamRoleLambdaExecution role would not exist in the compiled cloud formation and the deploy would fail with Template format error: Unresolved resource dependencies [IamPolicyLambdaExecution] in the Resources block of the template

How did you implement it:

This builds upon #3083 as it enables the custom role to be referenced directly without using the Fn::GetAtt syntax.

How can we verify it:

Deploy any serverless.yml with an event stream using only functions with custom roles (e.g. the sample in #3142 )

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[piohhmy]

Note: There may be a good refactoring opportunity here to keep things DRY. The code at https://github.com/serverless/serverless/blob/master/lib/plugins/aws/deploy/compile/functions/index.js#L32 is suspiciously similar to the if statement touched in this PR.

[pmuens]

Thanks for submitting a fix @piohhmy 👍

I was able to reproduce #3142 locally and this PR fixes the issue 🎉

Merging...