Adds VPNaaS support for OVN.
Add a new stand-alone VPN agent to support OVN+VPN. Add OVN-specific
service and device drivers that support this new VPN agent. This will
have no impact on the existing VPN solution for ML2/OVS, the existing
L3 agent and its VPN extension will still work.
Add a new VPN agent scheduler that will schedule VPN services to VPN
agents on a per-router basis.
Add two new database tables: vpn_ext_gws (to store extra port IDs)
and routervpnagentbindings (to store VPN agent ID per router).
More details see spec (neutron-specs/specs/xena/vpnaas-ovn.rst).
This work is based on work of MingShuan Xian (xianms@cn.ibm.com),
see https://bugs.launchpad.net/networking-ovn/+bug/1586253
Depends-On: https://review.opendev.org/c/openstack/neutron/+/847005
Depends-On: https://review.opendev.org/c/openstack/neutron-tempest-plugin/+/847007
Closes-Bug: #1905391
Change-Id: I632f86762d63edbfe225727db11ea21bbb1ffc25
256464aea6