Add dependency on Kubernetes client library when Kubernetes is enabled

Author: ants

setup.py

@@ -25,7 +25,7 @@
 
 EXTRAS_REQUIRE = {'aws': ['boto3'], 'etcd': ['python-etcd'], 'etcd3': ['python-etcd'],
                   'consul': ['python-consul'], 'exhibitor': ['kazoo'], 'zookeeper': ['kazoo'],
-                  'kubernetes': [], 'raft': ['pysyncobj', 'cryptography']}
+                  'kubernetes': ['kubernetes'], 'raft': ['pysyncobj', 'cryptography']}
 
 # Add here all kinds of additional classifiers as defined under
 # https://pypi.python.org/pypi?%3Aaction=list_classifiers