Add an attribute reporting client PKINIT-capable servers
    
    A new multi-valued server attribute `pkinit_server` was added which
    reports IPA masters that have PKINIT configuration usable by clients.
    
    The existing tests were modified to allow for testing the new attribute.
    
    https://pagure.io/freeipa/issue/6937
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>