allow specifying docker_volume_size on cluster-create

[Ricardo Rocha]

The docker_volume_size field can only be set on the cluster template. This is a value that users often want to override when using predefined and public cluster templates.

Blueprint https://blueprints.launchpad.net/magnum/+spec/flatten-attributes should solve this issue, but it requires a significant refactoring of the magnum code which makes the timeline for it to be done unclear.

In the meantime we should add this parameter explicitly on the cluster-create command so that users can already benefit from this.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/471766
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=79039bb4198b08500f5d5738b464d187f494a73a
Submitter: Jenkins
Branch: master

commit 79039bb4198b08500f5d5738b464d187f494a73a
Author: Ricardo Rocha <email address hidden>
Date: Wed Jun 7 13:10:33 2017 +0000

    Allow docker_volume_size on cluster create

    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