Adds support for the Indigo Virtual Switch (IVS)

Implements: blueprint ivs-interface-driver

Adds the IVS VIF type to portbindings.py.
Adds the VIF interface class to allow agents to bind to VIF switches.
Adds support to the BigSwitch plugin to request the IVS VIF type on nova compute nodes.
Adds unit tests for new interface class and changes to BigSwitch plugin.

Change-Id: I2fe104000523c60097c22946b0a80dc404dfe273
Merge "Adds support for the Indigo Virtual Switch (IVS)"