Split ZooKeeper and Exhibitor

Originally Exhibitor was supported in the ZooKeeper class and
configuration for Exhibitor was taken also from `zookeeper` section in
the yaml config file. In fact, Exhibitor just extends ZooKeeper and now
it is reflected in the code and also Exhibitor got it's own section in
the config.yaml file. It will make it easier to configure Exhibitor
hosts and port via environment variables when PR#211 will be merged.

postgres0.yml

@@ -23,13 +23,13 @@ etcd:
 #  hosts:
 #    - 127.0.0.1:2181
 #    - 127.0.0.2:2181
-#  exhibitor:
-#    poll_interval: 300
-#    port: 8181
-#    hosts:
-#      - host1
-#      - host2
-#      - host3
+#exhibitor:
+#  poll_interval: 300
+#  port: 8181
+#  hosts:
+#    - host1
+#    - host2
+#    - host3
 postgresql:
   name: postgresql0
   scope: *scope