Use the stack-name in the Name property of the ContainerDefinition

[arunasank]

Fixes #213

Currently, the options.service variable is required to be a string since it's concatenated with the options.prefix to create the Name property of the container. This PR removes that requirement and allows the options.service to be a Cloudformation reference. Besides the flexibility that this allowance gives us, it is also useful for differentiating containers of different stacks of the same repository, since the options.service can be referenced to AWS::StackName now, for example.

[arunasank]

Decided to remove the options.service dependency altogether from the Name property of the container, since the options.service property if also used to derive the name of the ECR repository for a stack. Using the cf.stackName reference directly in the Name property, to avoid clashing with the options.service.

[arunasank]

Additional tests are not required, since the changes use a cloudformation reference in the container name (and don't modify the datatype of the options.service variable as intended earlier), and the fact that the existing tests in template.validation.test.js pass is enough proof that the changes don't introduce validation errors in the cloudformation template. I've also tested this on a watchbot stack and double-confirmed that there are no validation errors, and that the name of the container is as expected.

[arunasank]

cc/ @mapbox/platform-engine-room

[jakepruitt]

Looks great!