crontab: ensure the script is run with shell

Some distros does not provide a default shell for Ceilometer user.
We can run the crontab by force shell usage and avoid running
issues.

Closes-bug: #1400523
Change-Id: I3a4d5c958a4620335d062baecfc362fd758768b6
(cherry picked from commit a83673c3bae7d560680cb77f2a2cc8c998fac2b7)