The ovn.ini file is a hold-over from the networking-ovn
tree. The docs all reference configuring OVN (and OVS)
options in ml2_conf.ini, so remove the old file and add
the neutron.ml2.ovn namespace to
etc/oslo-config-generator/ml2_conf.ini.
Trivialfix
Change-Id: I26dedc80e07aedffb1713560d4431b7a334b70b5