MXS-1220: Add deletion of listeners

The listeners can now be deleted via the REST API. Documented the added
endpoint in the REST API documentation.

server/core/config_runtime.cc

@@ -806,6 +806,7 @@ bool runtime_destroy_listener(SERVICE *service, const char *name)
         else
         {
             MXS_ERROR("Failed to destroy listener '%s' for service '%s'", name, service->name);
+            runtime_error("Failed to destroy listener '%s' for service '%s'", name, service->name);
         }
     }