MXS-862: Add authenticator options and instances

Authenticators now have a similar mechanism to the `router_options`
parameter which enables configurable authentication.

The authenticators also have a new initialize entry point which is similar
to the createInstance entry point of the filters and routers. The value of
`authenticator_options` is passed as a parameter to this function. The
return vaulue of the `initialize` entry point is passed to the `create`
entry point.

server/modules/authenticator/mysql_auth.c

@@ -62,6 +62,7 @@ static int mysql_auth_load_users(SERV_LISTENER *port);
  */
 static GWAUTHENTICATOR MyObject =
 {
+    NULL,                             /* No initialize entry point */
     NULL,                             /* No create entry point */
     mysql_auth_set_protocol_data,     /* Extract data into structure   */
     mysql_auth_is_client_ssl_capable, /* Check if client supports SSL  */