Kiali: Grafana password override handling

Currently the grafana password used for Kiali access is hardcoded in the package to prom-operator. While a customer can override the spec with values to provide their password, we can also do a simple values lookup to get the overridden password from monitoring.values.grafana.adminPassword. This would NOT cover the case where the password is changed via the UI - so may be worth checking if there is a secret we can use instead.