Layer of abstraction for commands that take an 'env' arg.

[jessicayuen]

There is a large subset of functionalities existing in cmd/root.go that are used only by commands that take an env argument, ex: ksonnet apply dev, ksonnet diff dev.

Each have a similiar workflow of having to modify the --cluster flag to point to the server the environment is located at, and what not.

All this common functionality is currently located in cmd/root.go and too tightly coupled with commands that have no notion of environments. There needs to be a layer of abstraction added for environment-using commands.

[jessicayuen]

This issue was moved to ksonnet/ksonnet#8