Improve unit test coverage for Cisco plugin nexus code

Improve unit test coverage for Cisco plugin nexus code

Closes-Bug: #1190622

This fix improves the unit test coverage for the Cisco Nexus plugin's
cisco_nexus_network_driver_v2.py and cisco_nexus_plugin_v2.py
source files from:
cisco_nexus_network_driver_v2: 72%
cisco_nexus_plugin_v2:         79%
To:
cisco_nexus_network_driver_v2: 99%
cisco_nexus_plugin_v2:         94%

Much of what the remaining "uncovered" code (coverage tool
reports as partially covered) can be attributed to the
coverage tool not being aware that execution does not
continue at the end of a save_and_reraise_exception()
context block (i.e. the exception will be reraised, but
the coverage tool isn't aware of this).

This fix and coverage results are dependent on the fix for #1246080.

Change-Id: Ie0e11fc0a12502739fb39bb4b30deb04dd31b7b0