Merge containerize into master

The containerize release modularizes the Deis PaaS into a series of Docker containers.  In order to facilitate this transition, some changes to the core Deis API were required.  This PR includes those changes, as well as the use of git submodules under `images` where the Docker image components are stored.

Please note this PR does not include updates to documentation.  Those will be forthcoming in a separate PR.