private JsonObject getVariables() {
            return config.getJsonObject(VARIABLES, new JsonObject());
        }
Gets all properties in the engine configuration that are prefixed
with the given prefix.
@param prefix the prefix
@return all prefixed properties
gateway/platforms/vertx3/vertx3/src/main/java/io/apiman/gateway/platforms/vertx3/common/config/VertxEngineConfig.java