Add docker-py integration tests aginst the docker daemon

This clones and run the integration tests for docker-py master as part
of the integration tests created on master.  docker-py hits the api
directly and should be a good way to identify regressions in the api.

The tests will be pinning to a specific tag and updated when a new version of docker-py is released so that we track releases and not master to ensure API compatibility.

 
Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Ouch.  We should definitely be using either `$DEST/docker-py-src` or something like that for this, or use `mktemp -d`.