Add vhost-user support via ovs capabilities/datapath_type

Add vhost-user support via ovs capabilities/datapath_type

Adds the ovs 'config' property which returns the contents of the
single row of the Open_vSwitch table. This gives access to certain
OVS capabilities such as datapath_types and iface_types.

Using this information in concert with the datapath_type config
option, vif details are calculated by the OVS mech driver. If
datapath_type == 'netdev' and OVS on the agent host is capable of
supporting dpdkvhostuser, then it is used.

Authored-By: Terry Wilson <twilson@redhat.com>
Co-Authored-By: Sean Mooney <sean.k.mooney@intel.com>
Closes-Bug: #1506127
Change-Id: I5047f1d1276e2f52ff02a0cba136e222779d059c