network api : reload : don't allow openswitch at all
ifupdown2 reload can't work with openswitch until we implement
ovs.
I don't think that too much users are mixing ovs && bridge anyway.
It's possible to use ifupdown2 with ovs for ifup/down with ifupdown script,
but config need to be changed, and I don't have tested too much.
(maybe add a conflict in ifupdown2 package with openvswitch package for now)