Nuage - Support template as an extended attribute for subnet and router

Nuage's VSP gives nice ability to users to define subnet and router templates. One can instantiate subnet and router than based on this template to inherit most if not all template properties. This is a very useful feature for all Nuage's current and future customers. Defining template as a resource itself is out of the scope here. This is an effort to extend current nuage plugin to accept precreated template as a parameter while creating subnet and router.