Now neutron provides only ovs based mechanism driver since linuxbridge
driver was removed. The null driver is intended for testing and
the openvswitch interface driver may be the only driver used in
real deployments (unless an external plugin is used).
Updating the default so that the option becomes optional, which may
help users deploy neutron more simply.
Change-Id: I0510b76d38168c06468856bfb58c7d561b14868b
f92c677b56