delete entries on "neutron agent-list" when charm removed or enable-local-dhcp-and-metadata=False

[Alvaro Uria]

When neutron-openvswitch is deployed, new entries will be added to "neutron agent-list".

However, removing neutron-openvswitch from several units won't reconfigure the neutron agents (ie. by deploying a different application of nova-compute with a new subordinate application of neutron-openvswitch charm).

In case that new subordinate has enable-local-dhcp-and-metadata=False, old neutron agents will be listed as down (xxx) and admin-state-up True.

I'd expect the charm to communicate to neutron-api charm that the agents should be removed (or disabled).

[James Page (james-page)]

agents should probably be removed automatically, however I don't think the neutron-openvswitch charm or the neutron-gateway charms have direct access credentials for the Neutron API so we need to come up with a neat way to make that actually happen.