iscsi: Iscsi rescan cleanup

* Remove forceIscsiScan(): iscsiadm already does that when refreshing
  sessions
* Cleanup iscsi.rescan(): Replace complex logic with a simpler
  implementation since new iscsiadm takes care of all of this itself
* Make iscsi.rescan() more testable
* Test confusing timeout semantics of iscsi.rescan()

Bug-Url: https://bugzilla.redhat.com/964595
Change-Id: I842eb37cea3bd5e8cd357a310dd966081b242abd
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21136
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Yaniv Bronhaim <ybronhei@redhat.com>