Add in-instance docker software config hook

This hook uses 'docker-py' library to deploy containers
for a container-manifest embeded in software-config 'config'
property.

Container manifest uses standard google container manifest schema[1].

WARNING: This is an experimental implementation.

[1]https://cloud.google.com/compute/docs/containers/container_vms.

Change-Id: I5ca30e99be1a247daf89c46aaa5a5034f1991379
Closes-Bug: #1380886
Merge "Add in-instance docker software config hook"