Enable volume and mount functionality

This commit enables the following functionality:
* Specify dataVolumes when creating a contianer to have Docker create data
volumes.
* Specify dataVolumesFrom when creating a container to mount the volumes
from the specified container in the container you're creating.
* Represent Docker data volumes inside the Rancher API as volumes and
mounts. A volume represents a volume from the host's perspective and a
mount describes the association between a volume and an instance.