change 'Host' object's 'host' attribute to 'host_name'

The function __repr__ of Host class which is located in hosts.py try to
access 'host' attribute, source code is "self.host",But in fact, Host object
don't has this attribute, it should be 'host_name'! We can find this attribute
from nova/api/openstack/compute/contrib/hosts.py file.

So I simply change 'self.host' to 'self.host_name'.

Change-Id: Ie76ba04da7592a596ab728fec981e9a1dbcc6a5f
Fixes: bug 1210043

novaclient/v1_1/hosts.py

@@ -21,7 +21,7 @@
 
 class Host(base.Resource):
     def __repr__(self):
-        return "<Host: %s>" % self.host
+        return "<Host: %s>" % self.host_name
 
     def _add_details(self, info):
         dico = 'resource' in info and info['resource'] or info