Validate VDSM user's sudoer permissions

On startup VDSM will try to run SUDO arbitrary command to validate that vdsm
sudoer configuration (sudoers.d/50_vdsm file) is set and read properly.
If not, will report to syslog and exit.

Change-Id: I1b1cb66e927f7ed0fcdd1898edd59730322cdeb1
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1024236
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/22888
Reviewed-by: Dan Kenigsberg <danken@redhat.com>