Add configurations for the OpenFlow Switch mechanism driver

This patch supports configurations for an environment of the OpenFlow Switch mechanism driver
Set the following variables in a localrc to be ran this mechanism driver.

Q_ML2_PLUGIN_MECHANISM_DRIVERS=ofswitch
Q_AGENT=ofswitch

Implements: blueprint ryu-ml2-driver
Change-Id: I774da9a26f241487dfa4ec124b12f528704d860b