As described in [1], the metadata with OVN uses a new neutron agent called
'neutron-ovn-metadata-agent'. In fact, the system service is also called
'neutron-ovn-metadata-agent', so it is more reasonable to modify its name.
[1] https://docs.openstack.org/networking-ovn/latest/contributor/design/metadata_api.html
Change-Id: I30d0eeee51ef3352cfb21f86138d60ad35f2b6fa