testrunner: capture warnings in log only if supported

Change-Id: Id9fbf6f359c415ba378d07cb4ed6d66324523412
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34460
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Saggi Mizrahi <smizrahi@redhat.com>
Reviewed-by: mooli tayer <mtayer@redhat.com>
Tested-by: mooli tayer <mtayer@redhat.com>