Use Dogtag REST API for certificate requests
    
    The Dogtag REST API gives better responses statuses than the RPC API
    and properly reports failure due to disabled CA (status 409).  Make
    'ra' extend 'RestClient' and refactor the 'request_certificate'
    method to use Dogtag's REST API.
    
    Part of: https://fedorahosted.org/freeipa/ticket/6260
    Part of: https://fedorahosted.org/freeipa/ticket/3473
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>