Cleanup of variables, queries and tests to enable showMultiCluster=True

Cleanup of variables, queries and tests to enable showMultiCluster=True

Rendering the dashboards with showMultiCluster=True allows for
them to work with multiple clusters storing their metrics in a single
Prometheus instance. This works via the cluster label and that functionality
already existed. This just fixes some inconsistencies in applying the label
filters.

Additionally this contains updates to the tests to have them succeed with
with both configurations and avoid the introduction of regressions in
regards to multiCluster in the future.

There also are some consistency cleanups here and there:
 * `datasource` was not used consistently
 * `cluster` label_values are determined from `ceph_health_status`
 * `job` template and filters on this label were removed to align multi cluster
    support solely via the `cluster` label
 * `ceph_hosts` filter now uses label_values from any ceph_metadata metrici
    to now show all instance values, but those of hosts with some Ceph
    component / daemon.
 *  Enable showMultiCluster=True since `cluster` label is now always present,
    via https://github.com/ceph/ceph/pull/54964

Improves: https://tracker.ceph.com/issues/64321
Signed-off-by: Christian Rohmann <christian.rohmann@inovex.de>