MB-13183: Remove millisec field in time format

%f or %s only exists in python 2.6 and later.

Change-Id: Ia2e6ef9cb2b06c7fb9d6bb51997d74370c7c2cdc
Reviewed-on: http://review.couchbase.org/45754
Reviewed-by: Bin Cui <bin.cui@gmail.com>
Tested-by: Bin Cui <bin.cui@gmail.com>

pump.py

@@ -459,7 +459,7 @@ def skip(self, key, vbucket_id):
 
     def get_timestamp(self):
         #milliseconds with three digits
-        return time.strftime("%Y-%m-%dT%H:%M:%S.%3s", time.gmtime())[:-7] + "Z"
+        return time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
 
     def add_counter(self, key, val=1):
         self.cur[key] = self.cur.get(key, 0.0) + val