[MONIT-4147] Make tenant credentials mandatory

With this patch the chart will fail to install if either tenant.name
or tenant.password are not set if they're necessary, example:

```
λ helm -n monitoring diff upgrade kubernetes-monitoring . -f values.yaml -f ibarrien.yaml
Error: Failed to render chart: exit status 1: Error: execution error at (cern-it-monitoring-kubernetes/templates/fluentbit-metrics/configmap.yaml:22:13): Tenand password is required
```

If no fluentbit forwarding is configured, i.e.:

```yaml
metrics:
  fluentbit:
    enabled: false

logs:
  fluentbit:
    enabled: false
```

then the chart installs fine as they're note required in that case.