spec: Require iscsi-initiator-utils >= 6.2.0.873-21

From version 6.2.0.873-7 up to version 6.2.0.873-20, there is a
regression in iscsi-initiator-utils, which cause creation of iscsi
storage domain to fail.

Vdsm requires now version 6.2.0.873-21 which fixed this regression.

Change-Id: I14e30dce00c89ff2f4901e7e2651b2e17d9abdc2
Bug-Url: https://bugzilla.redhat.com/1076603
Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27570
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/27599