MXS-1929: Allow service creation at runtime

Services can now be created at runtime. The command to create services is
exposed in the REST API.

server/core/internal/config_runtime.h

@@ -275,6 +275,15 @@ MXS_MONITOR* runtime_create_monitor_from_json(json_t* json);
  */
 MXS_FILTER_DEF* runtime_create_filter_from_json(json_t* json);
 
+/**
+ * @brief Create a new service from JSON
+ *
+ * @param json JSON defining the service
+ *
+ * @return Created service or NULL on error
+ */
+SERVICE* runtime_create_service_from_json(json_t* json);
+
 /**
  * @brief Alter a monitor using JSON
  *