tests: Add utilities for docker testing

tests: Add utilities for docker testing

docker.py is added with a number of useful subcommands to manager docker
images and instances for QEMU docker testing. Subcommands are:

run: A wrapper of "docker run" (or "sudo -n docker run" if necessary),
which takes care of killing and removing the running container at
SIGINT.

clean: Tear down all the containers including inactive ones that are
started by docker_run.

build: Compare an image from given dockerfile and rebuild it if they're
different.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-id: 1464755128-32490-2-git-send-email-famz@redhat.com