KC credentials not cleared on re-installations

[r-casula]

This is not necessarily a common use case, but it can happen and we should cover it.
After uninstalling the chart and reinstalling it, if the DB is not cleared, a subsequent installation of the chart will have some conflicts, especially with keycloak.

KC saves encrypted password in a table called credential in the kc_fndb db. We could clear all contents of that table or just the admin user, so that the kc-init-job can and kc itself can initialize properly.