Ensure log file for mobile data job exists

The mobile data job's log file's specification failed to ensure that
the log file is present. Hence, the log file would have been created
by the cron job. That however, would fail due to /var/log's
permissions. Hence, we now ensure that the file is present beforehand,
so puppet gets a chance to apply ownerships to it.

Bug: 66600
Change-Id: I7ea917ce58f6673aeb1589b092dc7a029b02fca5