[DO_NOT_MERGE] Support for user-defined environment variables

Support for user-defined environment variables set in an application deployment scope. Basically the user will be able to set environment variables during app creation, cartridge addition or even on existing applications. Environment variables set by the user will be persistent and replicated through all gears of the app, including new gears added in scaling events for example.

https://trello.com/c/iD81JIcZ/50-8-implement-the-rhc-environment-variable-support-carteco
https://trello.com/c/cWnktaUw/66-5-environment-variables-can-be-set-via-the-cli

Note: requires the appropriate REST API. Feature tests removed until the integration is complete.