Suggest running 'gnt-cluster upgrade --resume' at reboots

In the example crontab, add an entry to run 'gnt-cluster upgrade --resume'
upon reboot of the node, as suggested in the design document.
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Michele Tartara <mtartara@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
+
 # Restart failed instances (every 5 minutes)
 */5 * * * * root [ -x @SBINDIR@/ganeti-watcher ] && @SBINDIR@/ganeti-watcher