[OVN] VMs cannot access metadata when connected to a network with only IPv6 subnets

VMs cannot access the metadata service when connected to a network with only IPv6 subnets.

Neutron branch: master

Steps to reproduce:

1) Create a network with a single IPv6 subnet:

$ openstack network create ipv6-net-dhcpv6-slaac
$ openstack subnet create --subnet-range fdba:e036:9e22::/64 --ip-version 6 --gateway dba:e036:9e22::1 --ipv6-ra-mode slaac --ipv6-address-mode slaac --network ipv6-net-dhcpv6-slaac ipv6-subnet-dhcpv6-slaac

2) Create a VM using this network:

$ openstack server create --key-name my_key --flavor m1.small --image ubuntu-20.04-minimal-cloudimg-amd64 --network ipv6-net-dhcpv6-slaac --security-group sg1 my-vm-slaac

3) The following message is added to the metadata agent log file:

Jun 14 22:00:32 central neutron-ovn-metadata-agent[89379]: DEBUG neutron.agent.ovn.metadata.agent [-] No valid VIF ports were found for network 191a0539-edbc-4037-b973-dfa77e3208f6, tearing the namespace down if needed {{(pid=89379) _get_provision_params /opt/stack/neutron/neutron/agent/ovn/metadata/agent.py:720}}

which is produced here:

https://github.com/openstack/neutron/blob/79b2d709c80217830fed8ad73dcf6fbd3eea91b4/neutron/agent/ovn/metadata/agent.py#L719-L723

4) When an IPv4 subnet is added to the network and the VM is recreated, the metadata service is accessible to it over IPv6:

$ openstack subnet create --network ipv6-net-dhcpv6-slaac ipv4-subnet-dhcpv6-slaac --subnet-range 10.2.0.0/24
$ openstack server delete my-vm-slaac
$ openstack server create --key-name my_key --flavor m1.small --image ubuntu-20.04-minimal-cloudimg-amd64 --network ipv6-net-dhcpv6-slaac --security-group sg1 my-vm-slaac

From the VM:

ubuntu@my-vm-slaac:~$ curl http://[fe80::a9fe:a9fe%ens3]
1.0
2007-01-19
2007-03-01
2007-08-29
2007-10-10
2007-12-15
2008-02-01
2008-09-01
2009-04-04
latest

ubuntu@my-vm-slaac:~$ curl http://[fe80::a9fe:a9fe%ens3]/openstack
2012-08-10
2013-04-04
2013-10-17
2015-10-15
2016-06-30
2016-10-06
2017-02-22
2018-08-27
2020-10-14
latest

How reproducible: 100%