Improve code coverage of plugin module
Improve code coverage of plugin module

Added new unit tests to improve code coverage for create_vnf,
create_vnf_scale and create_vnf_trigger methods.

Change-Id: I3c57b4206b6f732b401701172edc3dbfe9c5a3ea

openstack/tacker,stackforge/tacker,stackforge/tacker,openstack/tacker,openstack/tacker