Scale up from single unit fails if leader gets removed before new unit settle

[paulomach]

Steps to reproduce:

1. Deploy 1 unit of mysql
2. Relate mysql and wordpress-k8s charm together
3. Add another unit for mysql charm and delete the leader unit.
4. Leader is elected on the mysql charm as the newly created unit without it being fully initialized
5. WordPress tries to connect to uninitialized unit and fails

[github-actions]

https://warthogs.atlassian.net/browse/DPE-2457

[paulomach]

Problem assumption:

While the joining unit has not yet joined the mysql cluster (data sync not finished), the leader unit is removed, the joining unit fails to become active with the data/users created for a previously existing relations.

Potential solution:

For a 2 unit application, when one of them get removed, ensure both are fully initialized before executing removal (by ways of defer or holding execution).