Add template attr. for subnet, router create in Nuage plugin

Add template attr. for subnet, router create in Nuage plugin

subnet create and router create commands now gives the ability
to the user to create subnets and routers via neutron cli with
additional attributes nuage_subnet_template and nuage_router_template
respectively. The instantiated subnet and router inherits the properties
of the template that was mentioned in the additional parameters.

Change-Id: Ib73313e840cb35ede89df858baa536aab883efa2
Closes-Bug: #1353112