Cluster downscaling fails with "FOREIGN KEY constraint failed"

[Martin Kalcok]

Executing `microovn cluster remove <MEMBER>` fails with:

Error: Delete "internal_cluster_members": FOREIGN KEY constraint failed

In addition to fixing this, there doesn't seem to be a code that would actually downscale the ovn-central service. When ovn-central member is being removed, it needs to be explicitly removed from the Northbound and Southbound database clusters, otherwise the remaining members will just consider it as "DOWN" and they'll expect it's return.
To properly remove ovn-central cluster member there are two options. Either the departing member executes

ovn-appctl cluster/leave OVN_Southbound
ovn-appctl cluster/leave OVN_Northbound

Or one of the remaining members must execute

ovn-appctl cluster/kick OVN_Southbound <member_id>
ovn-appctl cluster/kick OVN_Northbound <member_id>

The second option is more reliable as it works also for removing members that were already lost but it requires knowledge of the removed member ID in both Northbound and Southbound clustes.

[Martin Kalcok (martin-kalcok)]

PR merged: https://github.com/canonical/microovn/pull/35