util: fix execCmd to accept tuples

There was an issue occasionally when flags were given along with a
command in a tuple, there was an exception in execCmd, because +
operator was used to append tuple with commands to a list.

Change-Id: I8d1269d9a26b1fef1552976ae626cf4596471283
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26070
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>