Fix managedBy label for DNS zone

Even though managedBy output parameter was only used for failed host
managedBy memberships, it was defined in global baseldap.py
classes. Incorrect label was then being displayed also for DNS zone
per-zone permission attribute with the same name.

Move managedBy output parameter to host plugin. Define proper managedBy
output parameter in DNS plugin to improve clarity of this attribute.

https://fedorahosted.org/freeipa/ticket/2946

ipalib/plugins/host.py

@@ -172,6 +172,9 @@ host_output_params = (
     Str('revocation_reason?',
         label=_('Revocation reason'),
     ),
+    Str('managedby',
+        label=_('Failed managedby'),
+    ),
 )
 
 def validate_ipaddr(ugettext, ipaddr):