updating keystone preferred-api-version from v2 to v3 does not update credentials properly

[Andrea Ieri]

I'm filing this mostly as a reference, as this is probably too niche for anyone to care.

How to reproduce:
* deploy keystone using the v2 api
* deploy openstack-service-checks and relate it to keystone
* update keystone to use the v3 api

Result: the auth_url passed within the relation still contains v2
https://pastebin.canonical.com/p/H2xkcdW2MC/

Deploying openstack-service-checks /after/ keystone is already using the v3 api works fine.

This has been tested on openstack-service-checks v16.

[Alvaro Uria (aluria)]

Issue is that the keystone credentials are requested as soon as the "identity-credentials.available" relation is available, and data is stored in unitdata sqlite db ("keystonecreds" key).

"allow_keystone_store_overwrite" reactive function exists, but it is only called when "identity-credentials" is gone.

We need to investigate how to track identity-credentials-relation-changed hook. May be charm-interface-keystone-credentials code should track changes so we can monitor an extra state "identity-credentials.available.updated" or something similar):
https://github.com/openstack/charm-interface-keystone-credentials/blob/master/requires.py#L41,L61

[Alvaro Uria (aluria)]

Related to bug 1825951 (charm-interface-keystone-credentials)