Seems that a quite common openswitch basic configuration consist of a
one bridge, one port and one interface, all with the same interface
name. When performing such configuration in NetworkManager you need to
specify the slave-type for the ovs-interface, otherwise the master
interface specified there may match the bridge interface, resulting in
an error.
So, let's specify the slave-type for the ovs-interface, so that the
example will work also when the same interface name is specified for
both the ovs-bridge and the ovs-port.
https://bugzilla.redhat.com/show_bug.cgi?id=1638038
