XenServer: the cron job shouldn't print debug text into stderr
XenServer: the cron job shouldn't print debug text into stderr

one cron job attempts to print debug text into stderr; so the file
of /root/dead.letter gets created and its size grows continuously.
It could eventually threaten dom0 disk space. Maybe there are two
solutions: one is to redirect the output to a specific log file;
and rotate log files in the script. And the other one is simply
to redirect the output /dev/null. By considering the function of
this cron job and the printed contents are straight and simple,
this patch set goes with the later solution.

Change-Id: I4875e5e3837e6f0249e314c6c5f408c79145c6c1
Closes-Bug: 1503966

samgoon/devstack,openstack-dev/devstack,richliu/devstack,avvocatodemarchis/devstack,varunarya10/devstack,dirkmueller/devstack,dtroyer/devstack,Mahito/devstack,mssumanth/devstack,mssumanth/devstack,bigswitch/devstack,openstack-dev/devstack,noironetworks/devstack,prithivm/branch01,avvocatodemarchis/devstack,richliu/devstack,group-policy/devstack,dtroyer/devstack,thomasem/devstack,thomasem/devstack,bigswitch/devstack,group-policy/devstack,Mahito/devstack,samgoon/devstack,nawawi/openstack,noironetworks/devstack,bswartz/devstack,varunarya10/devstack,pombredanne/devstack,nawawi/openstack,pombredanne/devstack,bswartz/devstack,prithivm/branch01,dirkmueller/devstack