fix the bug that gnocchi dispatcher can't process single sample

When we use UDP to publish samples to Ceilometer-collector, it
always come one by one. So we have to check whether the gnocchi
dispatcher receive a single sample or a list.

Change-Id: Idb19c8b94681b827fdfcef9c0bbbf3b85688b23c
Closes-Bug:#1523124

ceilometer/dispatcher/gnocchi.py

@@ -235,6 +235,9 @@ class GnocchiDispatcher(dispatcher.MeterDispatcherBase):
                 return rd
 
     def record_metering_data(self, data):
+        # We may have receive only one counter on the wire
+        if not isinstance(data, list):
+            data = [data]
         # NOTE(sileht): skip sample generated by gnocchi itself
         data = [s for s in data if not self._is_gnocchi_activity(s)]