Add Dockerfile for containerizing enos

[ivotron]

This is what I do:

FROM python:2.7

RUN pip install enos && \
    pip install git+https://github.com/openstack/python-blazarclient

ENTRYPOINT ["enos"]

I can open a PR if you want.

[msimonin]

Good idea !
As you mentionned in #170, there will be maybe a need to pass some parameters through some environment variables. This is still unclear to me how to handle this right now.
At a first sight, I'd say we'll also need a enos user inside the container, /home/enos would be the working directory.