cacert_manage: fix CA cert renewal
    
    Since IPACertificate object is passed to CAInstance's
    update_people_entry() and update_authority_entry(),
    these are not be able to load this object as a certificate again and
    thus would fail. This commit fixes that.
    
    https://pagure.io/freeipa/issue/4985
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>