Add labels as an option during cluster create. If not given,
the default is taken from the cluster template.
Add labels 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: #1697651
Implements: blueprint flatten-attributes
Change-Id: I8990c78433dcbbca5bc4aa121678b02636346802