[OVN] The neutron_sync_mode conf is ignored for SB DB sync

[Lucas Alvares Gomes]

The neutron_sync_mode configuration option is ignored when syncing the OVN Southbound database, that class assumes that it's always in "repair" mode.

neutron_sync_mode can have 3 values:

* off, which should just turn off sync
* log, which should just log the inconsistencies found between the Neutron and OVN databases
* repair, which should repair all inconsistencies found between Neutron and OVN databases

But, right now only the Northbound DB sync class checks for these values: https://github.com/openstack/neutron/blob/dad8c3fecc4e97764a92f558d9df510fa2516877/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_db_sync.py#L98-L100

The Southbound DB sync does not even receive the sync mode as a parameter and assumes it's always in repair mode (even tho the default value for that option is "log"): https://github.com/openstack/neutron/blob/dad8c3fecc4e97764a92f558d9df510fa2516877/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_db_sync.py#L1330