Setup OVN network and its uplink

[luissimas]

Description

The charm should be able to automatically create an OVN network and its corresponding UPLINK network when the OVN relation is present.

The UPLINK network is used to provide external connectivity to OVN networks. Internally, this network is used by Incus to set the parameter external_ids:ovn-bridge-mappings on OVN (https://github.com/lxc/incus/blob/main/internal/server/network/driver_ovn.go#L1431). As far as I understand, this UPLINK network results in Incus configuring a network interface on the host to be used as a gateway. In this model, every Incus node is also an OVN gateway node.

It'd be nice to know if it is possible to have an Incus deployment in which only a subset of nodes act as a gateway. This seems possible in OpenStack, but I personally don't know the trade-offs involved.

Besides that, the UPLINK network might be of type bridge or physical. The operator should be able to choose which type to use, and the physical interface should be selected by using a Juju network binding.

Definition of done

• The charm exposes an extra binding ovn-uplink to select which physical network interface should be used to create the UPLINK network
• The charm exposes config options to configure several parameters on the UPLINK network such as IP ranges, the type of network etc
• When an OVN relation is present, the charm creates an OVN network with its associated UPLINK using the config options cited and adds that network to the default profile

Resources

https://linuxcontainers.org/incus/docs/main/howto/network_ovn_setup/
https://discuss.linuxcontainers.org/t/control-the-ovn-interface-creation-on-desired-cluster-member/19099