ipalib, ipaserver: fix incorrect API.register calls in docstrings
    
    Use API.add_plugin to load specific plugins into API objects. Use Registry
    to register plugins.
    
    This fixes doctests.
    
    https://fedorahosted.org/freeipa/ticket/4739
    https://fedorahosted.org/freeipa/ticket/5115
    
    Reviewed-By: David Kupka <dkupka@redhat.com>