Create an alias for `diego_docker` feature flag

[mike1808]

Issue

Since Cloud Controller NG now runs for both cf-for-k8s and cf-for-vms the diego_docker feature flag name makes no sense for cf-for-k8s. I suggest to make an alias for this flag called something like docker or container and also rewrite documentation for it so it becomes platform agnostic.

I'm happy to help to contribute this change.

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/175312319

The labels on this github issue will be updated when the story is started.

[sethboyles]

closing, as cf-for-k8s is deprecated