Support Nova availability zones

Extend API to support Nova availability zone selection for node groups
and node group templates.  This patch modifies the two provisioning
engines (direct and Heat) and includes changes to the DB schema.

DocImpact

Implements: blueprint support-nova-availability-zones
Change-Id: Id03848fd8d5949fa5ecbc494639fe59a70ddced3
Merge "Support Nova availability zones"