The OVN tree for several years has had an OVS submodule that pulls
in the version of OVS it is guaranteed to work with. There is
currently a patch in OVS master that breaks the build of OVN main
so switch to using the submodule.
Change-Id: I61195cd3ebf8a5015890d3ba3988c3d9795ee817
ee28e381fb