Configuration "sessionid" value

[joshmoore]

see: https://github.com/glencoesoftware/omero-ms-core/blob/master/src/main/java/com/glencoesoftware/omero/ms/core/OmeroWebSessionRequestHandler.java#L81

        Cookie cookie = event.getCookie("sessionid");
        if (cookie == null) {
            event.response().setStatusCode(403);
            event.response().end();
            return;
        }

For supporting multiple services on a single host, the cookie name is sometimes modified. The string sessionid should be overridable from the config.