tests: add-remove-cert: Use harcoded certificates instead of requesting them
    
    Requesting certificates for test purposes is not necessary as we allow to
    upload arbitrary certificate to the user, host or service. Also requesting
    certificate from dogtag takes some time and the test is slower for no good
    reason.
    More it's not posible to request certificate for stageuser even though it's
    possible to upload certificates to stageusers now.
    
    https://fedorahosted.org/freeipa/ticket/6623
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>