Prepare for multiple cisco ML2 mech drivers

Code tree reorganization in preparation for ML2 mechanism drivers for
other cisco products. The cisco nexus ML2 mechanism driver and its
test cases need to move down into their own subdirectory.

Closes-bug: #1277222

Change-Id: I2ba366332276069545b3deb0bbd39016a893327b
Merge "Prepare for multiple cisco ML2 mech drivers"