Add test cases for plugins/ml2/plugin.py
Add test cases for plugins/ml2/plugin.py

Test cases which generate MechanismDriverError failures and
validate correct behavior.

Partial-Bug: 1236127
Change-Id: I922e842b54add786723ecb7763d6b4f330794794

gkotton/vmware-nsx,gkotton/vmware-nsx