Move consul import to ConsulController class

Facilitates building on stretch where this module is not available

Index: patroni/features/environment.py
===================================================================
--- patroni.orig/features/environment.py
+++ patroni/features/environment.py
@@ -1,5 +1,4 @@
 import abc
-import consul
 import datetime
 import etcd
 import kazoo.client
@@ -372,6 +371,7 @@ class AbstractDcsController(AbstractCont
 class ConsulController(AbstractDcsController):
 
     def __init__(self, context):
+        import consul
         super(ConsulController, self).__init__(context)
         os.environ['PATRONI_CONSUL_HOST'] = 'localhost:8500'
         os.environ['PATRONI_CONSUL_REGISTER_SERVICE'] = 'on'