ra.get_certificate: use REST API
    
    Update ra.get_certificate to use the Dogtag REST API.  This change
    is being done as part of the Dogtag GSS-API authentication effort
    because the servlet-based method expects an internal Dogtag user.
    It is less intrusive to just change FreeIPA to call the REST API
    instead (which is also part of an existing ticket).
    
    Depends on https://pagure.io/dogtagpki/issue/2601 (which was merged
    and released long ago).
    
    Part of: https://pagure.io/freeipa/issue/3473
    Part of: https://pagure.io/freeipa/issue/5011
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>