MXS-1929: Allow startup with no services

MaxScale can now be started with an empty configuration file and services
can be created at runtime. Filters cannot yet be created at runtime so
complete runtime creation of configurations is not yet possible.

server/core/gateway.cc

@@ -2094,7 +2094,7 @@ int main(int argc, char **argv)
     /** Start the services that were created above */
     n_services = service_launch_all();
 
-    if (n_services == 0)
+    if (n_services == -1)
     {
         const char* logerr = "Failed to start all MaxScale services. Exiting.";
         print_log_n_stderr(true, true, logerr, logerr, 0);