Fix uncaught DB exception when trying to get a spare amphora

The MapLoadbalancerToAmphora task performs some database queries but
doesn't catch any exception from it.

In case something goes wrong with the database or the query, this commit
catchs any exception from the DB call, logs it and then returns None to
switch to the next decider branch (boot a new amphora).

Story 2007320
Task 38831

Change-Id: Ifb6c34426e0927534d332a8bbf2c66aac6c002c5

octavia/controller/worker/v1/tasks/database_tasks.py

@@ -529,10 +529,17 @@ class MapLoadbalancerToAmphora(BaseDatabaseTask):
         else:
             amp_az = CONF.nova.availability_zone
 
-        amp = self.amphora_repo.allocate_and_associate(
-            db_apis.get_session(),
-            loadbalancer_id,
-            amp_az)
+        try:
+            amp = self.amphora_repo.allocate_and_associate(
+                db_apis.get_session(),
+                loadbalancer_id,
+                amp_az)
+        except Exception as e:
+            LOG.error("Failed to get a spare amphora (AZ: {}) for "
+                      "loadbalancer {} due to: {}".format(
+                          amp_az, loadbalancer_id, e))
+            return None
+
         if amp is None:
             LOG.debug("No Amphora available for load balancer with id %s",
                       loadbalancer_id)