crontab: ensure the script is run with shell

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

Change-Id: Id222c943ac2f0957e1354a739ca42dde1162d1ae
(cherry picked from commit 8897ab59088b89770a6754bee7d94156922e19ee)