Support volume type in Sahara Node Group Template

Latest python-saharaclient (already in Heat dependencies) has ability to
specify cinder volume type when creating node group templates.
This patch adds support for this feature to
OS::Sahara::NodeGroupTemplate resource in Heat.

Change-Id: Ib12bc9b8da31fdb342b76fcb523212776ac2e802
Closes-Bug: #1369574
Merge "Support volume type in Sahara Node Group Template"