Make 'cluster-name' argument optional

[joshuabaird]

Describe the feature you are requesting Presently, a user has to specify --cluster-name as an argument to ensure that subnet tagging (and perhaps other features) work as expected. This forces users to tailor their application manifests to specific clusters which complicates the management of the manifests when a user needs to deploy to X number of clusters.

If possible, --cluster-name should be optional, and the default value should be inferred from some other means within the controller or via a global configuration exposed by the controller.

Motivation This will allow manifest authors to make author more generic manifests thus enabling them to deploy to X number of clusters more easily.

Describe the proposed solution you'd like Using the EC2 metadata service was my first thought here, but it doesn't look like the https://github.com/aws/containers-roadmap/issues/686 via this service yet (although it's said to be "coming soon").