Agent manager raises KeyError exception during load balancer sync

Issues:
Fixes F5Networks#1228

Problem: Agent manager can raise KeyError exception when syncing
pending load balancers.

Analysis: Added try/except block to catch KeyError exception and
log and error.

Tests: Tempest API tests.

f5_openstack_agent/lbaasv2/drivers/bigip/agent_manager.py

@@ -647,7 +647,11 @@ def _refresh_pending_services(self):
                     self.service_timeout(lb_id)
 
             if not lb_pending:
-                del self.pending_services[lb_id]
+                try:
+                    del self.pending_services[lb_id]
+                except KeyError as error:
+                    LOG.error("LB not found in pending services: {0}".
+                        format(e.message))
 
         # If there are services in the pending cache resync
         if self.pending_services: