Fix kubernetes demo files (#885)

- Update postgres docker image to the latest 11 version.

- Remove empty lines inside the `RUN` command to make the Dockerfile compatible with future docker versions.

- Set the `PATRONI_KUBERNETES_POD_IP` environment variable, which is required when _use_endpoints_ is enabled. Otherwise, the `KeyError` is raised [here](https://github.com/zalando/patroni/blob/master/patroni/dcs/kubernetes.py#L95).

- Set `EDITOR` environment variable to make configuration changes via `patronictl edit-config`.

kubernetes/openshift-example/templates/template_patroni_ephemeral.yml

@@ -108,7 +108,7 @@ objects:
       spec:
         containers:
         - env:
-          - name: POD_IP
+          - name: PATRONI_KUBERNETES_POD_IP
             valueFrom:
               fieldRef:
                 apiVersion: v1

kubernetes/openshift-example/templates/template_patroni_persistent.yaml

@@ -108,7 +108,7 @@ objects:
       spec:
         containers:
         - env:
-          - name: POD_IP
+          - name: PATRONI_KUBERNETES_POD_IP
             valueFrom:
               fieldRef:
                 apiVersion: v1