Removal of shared-db relation does not restore /etc/keystone/keystone.conf connection string

[Paul Goins]

This is related to #1797229, but with a slightly different nuance and, I think, lower importance.
(I've proposed a fix to the former which does not touch this issue, hence why I'm raising this separately.)

Prior to setting up a shared-db relation, the database connection string in /etc/keystone/keystone.conf reads as:

  connection = sqlite:////var/lib/keystone/keystone.db

However, after adding and then removing a shared-db relation, the connection string is left as it was with the database which was just removed, e.g.:

  connection = mysql://keystone:<password-redacted>@10.170.211.228/keystone

This connection string will get rewritten upon adding another shared-db relation, so I'm not sure whether we really care about this or not, as the juju app will go to an error state without a valid shared-db relation in place anyway.

Tested using the tests/bundles/bionic-rocky.yaml bundle within the keystone charm. I've also attached a crashdump captured from after removing the relation.

[Paul Goins (vultaire)]

Other information which might be relevant:

Host environment:
* OS: Ubuntu 18.10 cosmic
* juju: 2.5.1-cosmic-amd64
* lxd: 3.10

This was tested via an LXD controller.

[Ryan Beisner (1chb1n)]

Given that sqlite isn't actually a utilized db in a charm-deployed OpenStack, I'm inclined to recognize this bug as confirmed + wont-fix.