They could be useful to eliminate "unhealthy" pods from subsets addresses when the K8s service with label selectors are used.
Real-life example: the node where the primary was running has failed and being shutdown and Patroni can't update (remove) the role label.
Therefore on OpenShift the leader service will have two pods assigned, one of them is a failed primary.
With the readiness probe defined, the failed primary pod will be excluded from the list.
- 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`.
- It modifies the Dockerfile and entrypoint slightly to allow for OpenShift SCCs to operate correctly
- It adds 2 template examples that can be easily modified by changing parameters
Fixes#572
* Use ConfigMaps or Endpoins for leader elections and to keep cluster state
* Label pods with a postgres role
* change behavior of pip install. From now on it will not install all dependencies, you have to specify explicitly DCS you want to use Patroni with: `pip install patroni[etcd,zookeeper,kubernetes]`
