Don't allow host cn to be changed (it isn't used anyway).

We are required by LDAP schema to have a cn value. Don't let
users change it thinking they are actually doing something.

tickets 706 and 707

ipalib/plugins/host.py

@@ -533,6 +533,8 @@ class host_mod(LDAPUpdate):
 
     def pre_callback(self, ldap, dn, entry_attrs, attrs_list, *keys, **options):
         # Once a principal name is set it cannot be changed
+        if 'cn' in entry_attrs:
+            raise errors.ACIError(info='cn is immutable')
         if 'locality' in entry_attrs:
             entry_attrs['l'] = entry_attrs['locality']
             del entry_attrs['locality']