fix the docker address after implementing service provision is finished

[nimakaviani]

Provision assumes a call from the broker to a container management service (a docker daemon or CF / kubernetes). Once implementing Provision is on master, we need to look into how an actual deployment of the broker to CF would start the containers.

[nimakaviani]

PR #72 makes the address to the docker server configurable. It falls back to 127.0.0.1 in the absence of an env variable for DOCKER_SERVER but otherwise can be configured.