cert-request: raise CertificateOperationError if CA disabled
    
    Detect when cert-request returns HTTP 409, which indicates that the
    target CA is disabled - a valid scenario - and raise
    CertificateOperationError with a friendly message instead of
    HTTPRequestError.
    
    Fixes: https://fedorahosted.org/freeipa/ticket/6260
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>