certmonger: fix storing retrieved certificates
    
    This was a forgotten part from previous certificate refactoring which
    would cause issues since the second part of results throughout the
    dogtag-ipa-ca-renew-agent-submit is expected to be a string.
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>