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