crontab: ensure the script is run with shell

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

Change-Id: Ib05522d922fecfbd28aa8a8b092b4d3b47172d00
(cherry picked from commit c39fca28e68cbfdcfb64efb91ec9927a9385cdd3)