Check for gnt-cluster before running gnt-cluster upgrade

When ganeti is removed (not purged) `/etc/cron.d/ganeti` is not deleted,
thus after a reboot cron tries to execute gnt-cluster upgrade and fails.

The same pattern is used on all other cron entries.
Signed-off-by: Christos Trochalakis <christos@skroutz.gr>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>

doc/examples/ganeti.cron.in

 PATH=/sbin:/bin:/usr/sbin:/usr/bin:/usr/local/sbin:/usr/local/bin
 
 # On reboot, continue a Ganeti upgrade, if one was in progress
-@reboot root @SBINDIR@/gnt-cluster upgrade --resume
+@reboot root [ -x @SBINDIR@/gnt-cluster ] && @SBINDIR@/gnt-cluster upgrade --resume
 
 # Restart failed instances (every 5 minutes)
 */5 * * * * root [ -x @SBINDIR@/ganeti-watcher ] && @SBINDIR@/ganeti-watcher