Fix ovn-db-sync-util after removing l3 ovsdb connection
Fix ovn-db-sync-util after removing l3 ovsdb connection

As a result of d92e71c2 the sync util can fail because it tries to
look up the ovsdb connection from the mechanism_manager under 'ovn'
but the sync util registers under 'ovn-sync'.

Change-Id: I6998b08672761a501e427aab5d581064d9425dde
Closes-Bug: #1876752

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