Support 'delete_on_termination' with mounts

[hongbin]

Zun supports create/run containers with Cinder volume bind-mounts. In CLI, this is achieved by:

  $ zun run --mount source=<cinder_volume_id>,destination=<path_in_container> ...

or

  $ zun run --mount size=<cinder_volume_size>,destination=<path_in_container> ...

The first command mount an existing cinder volume to the container. The second command create a cinder volume with specified size and mount it to container.

This bug proposes to add an option to control how to circulate the volume if the container is deleted. For example:

  $ zun run --mount source=<cinder_volume_id>,destination=<path_in_container>,delete-on-termination=<True_or_False> ...

  $ zun run --mount size=<cinder_volume_size>,destination=<path_in_container>,delete-on-termination=<True_or_False> ...

If True, the cinder volume is deleted. If False, it is not deleted. The default should be False.