75859568 docker vs buildpack

For https://www.pivotaltracker.com/story/show/75859568. Adds a DockerPolicy to reject apps that specify both a docker_image and any buildpack (custom or admin). Completes the logic previously added to backends.rb to also consider admin buildpacks when checking if an app is ready for staging.

specs + CATs passing at time of PR. Not dependent on an other PR. Should bump cf-release.