Add master_flavor_id as an option during cluster create. If not given,
the default is taken from the cluster template.
Add master_flavor_id 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: #1699247
Change-Id: Id1d973167b381538121583a0a9691304b39e98de