vdsm-reg: collect vdsm-config.log

vdsm-config's log file was dumped in the catch-all /var/log directory.
Placing it under /var/log/vdsm-reg makes a nicer compartmentalization,
and makes vdsm's sosreport plugin collect it.

Change-Id: I124442ecc81fa1ff849ddd57af406a8a68c40f9e
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/25759
Tested-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>