Deprecated string.join() replaced

Deprecated string.join(execfn, " ")) was replaced by
' '.join(execfn)).

Change-Id: I8d16939a5d1d4518d1d91a525d8b403b3be12787
Signed-off-by: Petr Horáček <phoracek@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31897
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>