Add action to unregister neutron agent

[Martin Kalcok]

This is a followup bug based on comment in this bug https://bugs.launchpad.net/charm-nova-compute/+bug/1691998 . The comment mentions that when nova-compute unit is removed, neutron agent (that was originally subordinate charm of this unit) remains registered in the Openstack and needs to be removed manually.

Similar issue was solved in nova-compute charm by adding a action `remove-from-cloud` that unregisters nova-compute service from nova-cloud-controller which allows for cleaner removal of the unit.

I think that similar approach could be used in neutron-ovs charm.

[Frode Nordahl (fnordahl)]

This does indeed sound useful. Just wanted to point out that we have multiple SDN alternatives that might benefit from this, the traditional Neutron ML2/OVS, the Neutron ML2/OVN and possibly other third party offerings. So please do take reusability into account when implementing this, perhaps it makes sense to add the action to the neutron-api charm for everyone to use from the start?

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-neutron-openvswitch/+/808482

[Frode Nordahl (fnordahl)]

Alvaro pointed out OOB that having the action centralized is undesirable because in a large deployment it would be cumbersome for the end user to feed the correct agent names into action parameters to the neutron-api charm. If the n-ovs or other SDN charm initiates the operation it will be able to provide the correct agent names automatically.

Another suggestion was to host the logic within the neutron-api charm and have some inter-charm communication happen between SDN-charm and the neutron-api charm. The benefit of this approach is that we will avoid putting credentials where they may not currently be required.

However, for this to work we would have to figure out if this would work in a synchronous manner. I.e. the charm operator would want to run `juju run-action --wait SDN-UNIT/0 remove-agents; do the next thing after the agent has been removed`

[OpenStack Infra (hudson-openstack)]

Change abandoned by "Gabriel Angelo Sgarbi Cocenza <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/charm-neutron-openvswitch/+/808482