Make Patroni Kubernetes native (#500)

* 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]`

tests/test_config.py

@@ -49,6 +49,9 @@ class TestConfig(unittest.TestCase):
             'PATRONI_ETCD_CERT': '/cert',
             'PATRONI_ETCD_KEY': '/key',
             'PATRONI_CONSUL_HOST': '127.0.0.1:8500',
+            'PATRONI_KUBERNETES_LABELS': 'a:b:c',
+            'PATRONI_KUBERNETES_SCOPE_LABEL': 'a',
+            'PATRONI_KUBERNETES_PORTS': '[{"name": "postgresql"}]',
             'PATRONI_ZOOKEEPER_HOSTS': "'host1:2181','host2:2181'",
             'PATRONI_EXHIBITOR_HOSTS': 'host1,host2',
             'PATRONI_EXHIBITOR_PORT': '8181',