Add capability to only validate template in OSC plug-in
Add capability to only validate template in OSC plug-in

User may only want to validate template via heat-translator OpenStackClient
(OSC) plug-in. A new optional arg is added.

Partially Implements: blueprint template-validation-only

Change-Id: Ie6b4383e8505077f3b4a459f3003648c723aaa7a

openstack/heat-translator,openstack/heat-translator