Refactors based on discussion on Discourse

[johnsca]

See https://discourse.charmhub.io/t/kubernetes-service-interface-with-operator-framework/3782

[johnsca]

One change in this that's not relevant to the discussion on Discourse is that I started to find the k8s_service_available event name a bit off, so this is proposing to change it to k8s_services_changed. While that doesn't reflect the idea of the list first becoming populated, it's still true then that the list has "changed" (from being empty, previously) and the actual use-case for the event is to ensure that changes to the list do lead to updates to the pod spec, so I think it's more accurate. Open to discussion there, though. And, of course, this will require updating the charm code as well.

Also, I think it would be worth adding unit tests to this using the framework's testing harness.

[DomFleischmann]

I've tested the changes and I they LGTM. This also solves our issue with emitting the k8s_services_changed event.