API: add constants for key of template attributes
API: add constants for key of template attributes

Change-Id: I2a58663504616fc5fa55ffff04889fe69f3f4af0

SripriyaSeetharam/tacker,zeinsteinz/tacker,stackforge/tacker,priya-pp/Tacker,openstack/tacker,zeinsteinz/tacker,trozet/tacker,openstack/tacker,stackforge/tacker,openstack/tacker,priya-pp/Tacker,SripriyaSeetharam/tacker,trozet/tacker