MXS-1220: Add execution of module commands to REST API

The module command self links now point to an endpoint that executes the
module command. Depending on the type of the module command, either a GET
or a POST request must be made.

examples/roundrobinrouter.cpp

@@ -673,7 +673,8 @@ static MXS_ROUTER* createInstance(SERVICE* service, char** options)
      * the pointer. */
 
     /* Register a custom command */
-    if (!modulecmd_register_command("rrrouter", "test_command", custom_cmd_example,
+    if (!modulecmd_register_command("rrrouter", "test_command",
+                                    MODULECMD_TYPE_ACTIVE, custom_cmd_example,
                                     2, custom_cmd_args))
     {
         MXS_ERROR("Module command registration failed.");