ITM: Fix for tunnel ports not getting deleted.

ITM: Fix for tunnel ports not getting deleted.

During deletion of transport zone, tunnel ports (on ovs) connecting to
l2gw devices were not getting deleted.

Root cause: During deletion of transport zone, there was a logic fetching
transport zone from config DS which actually doesn't exist.

Fix: Changed the logic to pass transport zone object from
TransportZoneListener to the worker.

Change-Id: I27dc944c43b83f1b6c17cf9ccbacc76201243ed4
Signed-off-by: Somashekar Byrappa <somashekar.byrappa@ericsson.com>