svc_monitor unable to handle empty az list from nova

This fix adds error handling to svc_monitor code. If nova
returns empty az_list or if the call to nova fails for some
reason, svc_monitor will not crash after this fix.

Change-Id: Ib132401d931c763c36c9eca46bfd855663b070b9
Closes-Bug: #1593432

src/config/svc-monitor/svc_monitor/scheduler/vrouter_scheduler.py

@@ -59,17 +59,21 @@ def _get_az_vrouter_list(self):
         az_list = self._nc.oper('availability_zones', 'list',
                                 self._args.admin_tenant_name,
                                 detailed=True)
+        if not az_list:
+            self._logger.error("Failed fetching azs from nova")
+            return None
+
         az_vr_list = []
         for az in az_list:
-            if self._args.netns_availability_zone not in str(az):
-                continue
             try:
-                # check if the az is available
-                if not az.zoneState['available']:
-                    continue
-                # check if there are any hosts
-                if not az.hosts:
+                # check:
+                # 1. If az is mentioned in config
+                # 2. If the az is enabled & active
+                # 3. If it has any hosts
+                if az.zoneName not in self._args.netns_availability_zone or \
+                   not az.zoneState['available'] or not az.hosts:
                     continue
+
                 # check if hosts are active & enabled
                 for host,host_status in az.hosts.iteritems():
                     if (('nova-compute' in host_status) and \