grafana: Configure Loki datastore

Usually, Grafana datastores are configured using its web GUI.  When
setting up a datastore that requires TLS client authentication, the
client certificate and private key have to be pasted into the form.
For certificates that renew frequently, this method would require a
frequent manual effort.  Fortunately, Grafana supports defining
datastores via its "provisioning" mechanism, reading the configuration
from YAML files on the filesystem.

grafana/grafana.yaml

@@ -73,6 +73,9 @@ spec:
         - mountPath: /etc/grafana
           name: config
           readOnly: true
+        - mountPath: /etc/grafana/provisioning/datasources
+          name: datasources
+          readOnly: true
         - mountPath: /run/secrets/grafana
           name: secrets
           readOnly: true
@@ -86,6 +89,10 @@ spec:
       - name: config
         configMap:
           name: grafana
+      - name: datasources
+        configMap:
+          name: datasources
+          optional: true
       - name: grafana
         persistentVolumeClaim:
           claimName: grafana