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