Add docker-py integration tests aginst the docker daemon

Closes #9676

Changes from @crosbymichael's code:
- 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

```console
$ make test-docker-py
docker build -t "docker:add-docker-py-tests" .
Sending build context to Docker daemon 43.49 MB
Sending build context to Docker daemon 
Step 0 : FROM ubuntu:14.04
...
Successfully built c04cdc4c528a
docker run --rm -it --privileged -e BUILDFLAGS -e DOCKER_CLIENTONLY -e DOCKER_EXECDRIVER -e DOCKER_GRAPHDRIVER -e TESTDIRS -e TESTFLAGS -e TIMEOUT -v "/home/tianon/docker/docker/bundles:/go/src/github.com/docker/docker/bundles" "docker:test-docker-py" hack/make.sh binary test-docker-py

bundles/1.4.1-dev already exists. Removing.

---> Making bundle: binary (in bundles/1.4.1-dev/binary)
Created binary: /go/src/github.com/docker/docker/bundles/1.4.1-dev/binary/docker-1.4.1-dev

---> Making bundle: test-docker-py (in bundles/1.4.1-dev/test-docker-py)
+++ exec docker --daemon --debug --storage-driver vfs --exec-driver native --pidfile /go/src/github.com/docker/docker/bundles/1.4.1-dev/test-docker-py/docker.pid
...................................................
----------------------------------------------------------------------
Ran 51 tests in 77.919s

OK
++++ cat /go/src/github.com/docker/docker/bundles/1.4.1-dev/test-docker-py/docker.pid
+++ kill 685

```