Work around python-nss bug on unrecognised OIDs
    
    A bug in python-nss causes an error to be thrown when converting an
    unrecognised OID to a string.  If cert-request receives a PKCS #10
    CSR with an unknown extension, the error is thrown.
    
    Work around this error by first checking if the OID is recognised
    and, if it is not, using a different method to obtain its string
    representation.
    
    Once the python-nss bug is fixed, this workaround should be
    reverted.  https://bugzilla.redhat.com/show_bug.cgi?id=1246729
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>