Neutron ovs plugin should not require vswitch::ovs

I don't see any reason openvswitch components should be required on the same
machine than the neutron ovs *plugin*. It is definitely required for the
neutron ovs *agent* though.

Change-Id: I1d7128f8a79d788b48af4acf504d48a19e8fe12b
Merge remote-tracking branch 'upstream/master'