Port location tracking for BigSwitch Plugin

Adds a new table to the Big Switch plugin to keep track of
the nova compute node host IDs that ports reside on.
This table is then used to allow users to override
the VIF type for a compute node based on the host ID.
This allows quantum to control an environment with multiple
VIF types.

Change-Id: I63eb66d970650237aed2d5dc6676a6d097988f8d
Implements: blueprint hostid-vif-override
Merge "Port location tracking for BigSwitch Plugin"