Avoid warning when time taken is close to zero

We use "%(delay).2f" to print the time taken so even
if the delay is say 0.001, we will still print the
warning. We should just round it off till the 2nd place
to avoid odd looking lines that say:

run outlasted interval by 0.00 sec

Change-Id: I7137eb7ad985d7f35adc62a65ad1218a39a7a959

oslo_service/loopingcall.py

@@ -169,7 +169,7 @@ class FixedIntervalLoopingCall(LoopingCallBase):
 
     def start(self, interval, initial_delay=None, stop_on_exception=True):
         def _idle_for(result, elapsed):
-            delay = elapsed - interval
+            delay = round(elapsed - interval, 2)
             if delay > 0:
                 func_name = reflection.get_callable_name(self.f)
                 LOG.warning(_LW('Function %(func_name)r run outlasted '