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.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Tweak test-docker-py feature

- move docker/docker-py clone to the Dockerfile
- put "integration test daemon startup" code in a separate file for both scripts to source
- add new test-docker-py Makefile target
- include "python-websocket" package in Dockerfile for running the tests

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>
Merge pull request #9741 from tianon/test-docker-py

Add docker-py integration tests aginst the docker daemon