[OVN][FT] Also update nb_cfg along with nb_cfg_timestamp

FT ``test_agent_updated_at_use_nb_cfg_timestamp`` updates
"nb_cfg_timestamp" in Chassis_Private table and then
checks if agent cache get's updated to the same value.

But "ChassisAgentWriteEvent" which is responsible for
updating AgentCache doesn't get triggered with updates on
"nb_cfg_timestamp", so this patch proposes also to increment
"nb_cfg" by 1 to trigger the Event as in general scenario
timestamep get's updated along with nb_cfg.

Closes-Bug: #1974149
Change-Id: I8889746ff2ee2629ee7c38ac162ea7c4d9ee0734
(cherry picked from commit 1c89db4)

Author: karelyatin

neutron/tests/functional/plugins/ml2/drivers/ovn/mech_driver/ovsdb/test_ovsdb_monitor.py

@@ -470,6 +470,12 @@ def check_agent_ts():
         nb_cfg_timestamp = timestamp * 1000
         self.sb_api.db_set('Chassis_Private', self.chassis_name, (
             'nb_cfg_timestamp', nb_cfg_timestamp)).execute(check_error=True)
+        # Also increment nb_cfg by 1 to trigger ChassisAgentWriteEvent which
+        # is responsible to update AgentCache
+        old_nb_cfg = self.sb_api.db_get('Chassis_Private', self.chassis_name,
+            'nb_cfg').execute(check_error=True)
+        self.sb_api.db_set('Chassis_Private', self.chassis_name, (
+            'nb_cfg', old_nb_cfg + 1)).execute(check_error=True)
         try:
             n_utils.wait_until_true(check_agent_ts, timeout=5)
         except n_utils.WaitTimeout: