Moving panic function to utils and set panic mock for tests

The patch moves panic function to vdsm utils for general use.
During tests panic call will raise assertionError.

Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>
Change-Id: Ia2ae7ba76d6a935c0f8c8854bedce985376ed219
Reviewed-on: http://gerrit.ovirt.org/14427
Reviewed-by: Dan Kenigsberg <danken@redhat.com>