Add automatic setup/teardown of OVSDB manager

Add an extras.d script to automatically setup and teardown the OVSDB manager
connection when running stack.sh and unstack.sh if opendaylight is in the
list of MechanismDrivers for the ML2 plugin.

Change-Id: Ib91ed9d28c76e2006e5118e7632b21e3cdcef7a8