Change scsi_id command path to be configured at runtime

On fedora 18 scsi_id path is no longer under /sbin/scsi_id,
we configure vdsm to look for the path at runtime
and thus remove it from constants.

Change-Id: I409d4da0ba429564466271aded32e96f9401cd6c
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=886087
Signed-off-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/10824
Reviewed-by: Dan Kenigsberg <danken@redhat.com>