Description of problem:  https://review.openstack.org/#/c/299280/

When starting up, we don't want to delete the patch port between br-int
and br-tun unless we're also dropping the flows..  In liberty both of
these bridges were switched to not dump flows on startup and to put the
bridges in secure mode so that default flood flows are not installed
when the bridge is created.

Without this patch the patch port is torn down and not reinstalled until
br-tun is setup again.