On python <3.4 the separator for json.dump() defaulted to (', ', ': ')
which may cause trailing whitespaces in rendered JSON files when used
with indent. This change sets the separator to (',', ': ') to prevent
trailing whitespaces with older versions of python.
See https://docs.python.org/2/library/json.html#basic-usage
Change-Id: I81fabd54659bdb28da43fb4900f19dd94a70336c