OVN load balancer backend health checks

At this time OVN load balancers will not support health monitoring of backends due to limitations in the OVN implementation. This means that should one backend fail, the load balancer will fail in handling any requests destined for that backend.

The limitations in the current OVN health check system are:

Health checks must be associated with a logical switch port, meaning they must be tightly coupled to an instance NIC rather than target IP address. This is incompatible with our requirements.
Health checks seemingly do not work unless the source address of the health check is reachable inside the OVN network, as such it appears that a local port is required to be added to each OVN network with an IP for use as the healthcheck source.
Health checks don’t work with IPv6 backends.

See https://discuss.linuxcontainers.org/t/lxd-network-load-balancers-ovn/14317#further-information-12

canonical / lxd

OVN load balancer backend health checks #13769