Add environment overrides for app.yaml

[bryanl]

If a team has checked their ksonnet app into an environment, it would be desirable for a developer to have the ability to override environment settings to suit their local environments.

• env overrides live in .envoverride.yaml. The file will contain a map of environment specs.
• env override support will be present in the following env commands:
  • add
  • describe
  • list
  • rm
  • set
  • targets
• add a new filed to the environment specs to signify it is an override. The field will be called is_override.