feat(nextcloud): Make postgresql HA

This patch introduces an override to make PostgreSQL HA by running more
than one instance managed by the postgresql-operator.

This is a first experiments with the HA features and might become a
blue-print for further experiments.

apps/k8s01/nextcloud/database-override.yaml

+apiVersion: "acid.zalan.do/v1"
+kind: postgresql
+metadata:
+  name: nextcloud-postgres
+  namespace: nextcloud
+spec:
+  numberOfInstances: 2
+  volume:
+    size: 1Gi

apps/k8s01/nextcloud/kustomization.yaml

@@ -6,3 +6,5 @@ resources:
   - certificate.yaml
   - nextcloud-values.yaml
   - ../../../shared/resourcequotas/default.yaml
+patchesStrategicMerge:
+  - database-override.yaml