Bug 10379: silence zebra rebuild cron when no instances are enabled

Bug 10379: silence zebra rebuild cron when no instances are enabled

This adds a -q option to koha-rebuild-zebra that prevents it outputting
help if no instances are provided on the command line. This means that
if you've just installed the package, you won't get messages from cron
telling you how to use the script.

Test plan:
* install koha-common 3.12 or master onto a system with no enabled
  instances.
* note that you or the sysadmin will start getting emails every 5
  minutes.
* replace /etc/cron.d/koha-common and /usr/sbin/koha-rebuild-zebra with
  the versions from this patch.
* note that the emails stop flowing.

Signed-off-by: Magnus Enger <magnus@enger.priv.no>
"sudo koha-rebuild-zebra -q" gives no output, as expected.
Followed the test plan, all looks good.
Signed-off-by: Galen Charlton <gmc@esilibrary.com>