Rename bay to cluster

The use of new magnum term of bay has caused confusion with users. The common term is now cluster and magnum should be updated to follow the more common term for better user experience. Both python client and api service APIs will be updated to replace bay with cluster. Internal classes, documentation, and any references will be updated to the new term. Bay and BayModel APIs will still be valid with a depreciated warning to the user for the next few releases. See Newton Mid-cycle notes[1] for details behind this change.

Note (hongbin): This work needs to be backward-compatible, which means the bay and baymodel abstraction should work as before.

[1] https://etherpad.openstack.org/p/magnum-newton-midcycle-topics

changes:
* "bay(s)" to "cluster(s)"
  "Bay(s)" to "Cluster(s)"
* "baymodel(s)" to "clustertemplate(s)" for programming
* "Baymodel(s)" to "ClusterTemplate(s)" for programming
  insert one space between "cluster" and "template(s)" for user's eye (on browser view)