x509: fix SAN directoryName parsing
    
    The subjectAltName extension parsing code in ipalib.x509 fails on
    directoryName values because the Choice structure is not endowed
    with an inner type.  Implement the Name structure, whose inner type
    is a CHOICE { SEQUENCE OF RelativeDistinguishedName }, to resolve.
    
    Note that the structure still does not get fully parsed; only enough
    to recognise the SequenceOf tag and not fail.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6022
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>