[luacheck] Introduce ignores using luacheckrc

Allows the use of _ in loop variables in order to keep readability whilst ignoring the "unused loop variable" warning
Ignores globals which are required to setup unit tests

Author: talha-p

.luacheckrc

@@ -0,0 +1,4 @@
+ignore = {"213/_.*"} -- unused Loop Variables beginning with underscore
+files["openwisp-config/tests"] = {
+    ignore = {"11./Test.*"} -- globals relating to defining unittests
+}