Support availability zones in Node Group Templates

Latest python-saharaclient (already in Heat dependencies) has ability
to provide availability zones for servers and volumes when creating
node group templates.
This patch adds support for this feature to
OS::Sahara::NodeGroupTemplate resource in Heat.

Change-Id: Ic74fd43f5d8c692ddb40d0c6cefa5ed6327dd654
Merge "Support availability zones in Node Group Templates"