Feature/allow environment variables

Awesome plug-in, thanks!!

Couple things in this pull request for your consideration:

1. Added the ability to specify environment variables to a running container.
2. Updated the pull image logic to allow for image named "tutum/rabbitmq".  The previous code would fail this image with a 404 and I believe it's due to the Docker API v1.14 update of the fromImage query parameter.

Thanks again.