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]`

requirements.txt

@@ -1,10 +1,10 @@
-urllib3>=1.9
+urllib3>=1.19.1,!=1.21
 boto
-psycopg2>=2.6.1
+psycopg2>=2.5.4
 PyYAML
 requests
 six >= 1.7
-kazoo==2.2.1
+kazoo>=1.3.1
 python-etcd>=0.4.3,<0.5
 python-consul>=0.7.0
 click>=4.1
@@ -13,3 +13,4 @@ tzlocal
 python-dateutil
 psutil
 cdiff
+kubernetes==3.0.0