Allow CloudFormation action in CodePipeline to push stack outputs to an optional output artifact

[john-shaskin]

I'm being lazy and combining two changes into one PR here.

1. Adding ECR permissions to the roles used for CloudFormation and CodeBuild
2. Adding ability to optionally specify an output artifact name in a CloudFormation action, which will cause the stack output values to be pushed into a JSON file that is output for successive pipeline actions to access.

[brettswift]

I just created an issue that is less related to this PR and more to the work I'm doing, however because this code touches the same area (adding action_mode to the cfn step), I wanted to raise it here, even though I'm approving for merge.

#20

Ultimately, I think the best route might be to take items like the configuration object, and allow that to be overridden, instead of having to pick apart all the properties of it individually.. like action_mode, as we would inevitably end up with a constructor signature with 30 parameters.

We just need to figure out a direction after learning from writing a few different troposphere examples.