grafana: Avoid noise due to random admin password

We do not really set any specific admin password, so the Grafana chart generates a random one if none can be found in the Kubernetes secret using the Helm lookup template function.

This would be all good and nice, but the lookup function only works with --dry-run=server in Helm so at the moment we always get a new random value on every run, generating some worrying noise when running in --check --diff mode if you do not know what is going on.

Since I found no way to make lookup work with helm diff, let's retrieve the current value (if any) on the Ansible side.

See https://gitlab.collabora.com/sysadmin/ansible-core-services/-/merge_requests/60