[OVN][FT] Mech driver stop accessing DB at exit
[OVN][FT] Mech driver stop accessing DB at exit

"OVNMechanismDriver._clean_hash_ring" is called at exit. Because the
SQL connection is already closed, an exception is logged. This patch
mocks this method and avoids the OVN FTs to log this message.

Closes-Bug: #1919352

Change-Id: I6c68d15d0af644ab00467bea8e0cd3ca816f5cc5

mahak/neutron,mahak/neutron,openstack/neutron,mahak/neutron,openstack/neutron,openstack/neutron