Fixed timestamp creation in MongoDB mapreduce

datetime.strftime('%s') doesn't respect tzinfo see:
http://bugs.python.org/issue12750

Naive time in UTC was converted as a local time
causing test failures in timezone with DST

Change-Id: I0fb5afae91a3b1240402d252b8e3a4fe0497fa05
Merge "Fixed timestamp creation in MongoDB mapreduce"