Configuration option for rrd images directory

Problem/Motivation

From #48 in https://www.drupal.org/node/2771913#comment-11595095:

+++ b/modules/monitoring_rrd/monitoring_rrd.module
@@ -0,0 +1,610 @@
+  $img_folder_path = 'sites/default/files/monitoring_rrd_files/';

this should be a configuration option too.

Proposed resolution

Add a configuration option for setting up the RRD images directory (similar to the option RRD files directory).

Remaining tasks

User interface changes

API changes

Data model changes