GENIUS-254 GENIUS-119 GRE and L2GW tunnel missing

GENIUS-254 GENIUS-119 GRE and L2GW tunnel missing

There is a race condition between creation of tun port and
bridge ref, port not created. When we are trying to add bridge
to ovsdb from SB, NB is trying to add port for same bridge.
during which NB will check DS for bridge entry,which will be
empty as SB is trying to add it in ovsdb at the same time.
So we are adding eventcallback to synchronize both events.

Change-Id: I7f6c0583d90d5aa91e0ae063123680bdece08998
Signed-off-by: Nidhi <nidhi.adhvaryu@ericsson.com>