Add docker_volume_size as an option during cluster create. If not given,
the default is taken from the cluster template.
Add docker_volume_size in the Cluster object and use that instead
of the one from ClusterTemplate.
Update both magnum and magnum cli documentation to reflect the above changes.
Partial-Bug: #1697648
Implements: blueprint flatten-attributes
Change-Id: Ic6d77e6fdf5b068fa5319b238f4fd98b4d499be4