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/gssapi_backend_auth.c

@@ -170,6 +170,7 @@ static int gssapi_backend_auth_authenticate(DCB *dcb)
  */
 static GWAUTHENTICATOR MyObject =
 {
+    NULL,                               /* TODO: implement initialize entry point */
     gssapi_auth_alloc,                  /* Allocate authenticator data */
     gssapi_backend_auth_extract,        /* Extract data into structure   */
     gssapi_backend_auth_connectssl,     /* Check if client supports SSL  */