Tests: Fixed resource manager stress test

It now should perform more reliably and reduce side effects. Can be now
be run even on weaker hosts though the test will be less effective.

Change-Id: I1ba095724a251968c8d5b2767e6491825d79297e
Signed-off-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19252
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>