Improve unit test coverage for Cisco plugin model code

Improve unit test coverage for Cisco plugin model code

Closes-Bug: #1190620

This fix improves unit test coverage for:
quantum/plugins/cisco/models/virt_phy_sw_v2.py
Test coverage is improved from about 78% to 99%.

One change included in this fix is removal of some code in
the _invoke_plugin() method in virt_phy_sw_v2.py which looks
like it's attempting to handle the case where the number of
arguments being passed to _invoke_plugin() exceeds the number
of arguments expected for the target plugin method. This
section of code does not get executed for any existing
calls to _invoke_plugin(), and it doesn't appear that
this logic would work (except when the target plugin method
includes a **kwargs expansion).

Change-Id: Ibceb7a462a213f3ba693bcbe94e77d97b2e1440a