Fix UnboundLocalError in sqs_queue

The variable `existing_value` is nowhere to be found, but looks like
this might have been missed in a rename. Changing to `value`.

Fixes #172

plugins/modules/sqs_queue.py

@@ -375,7 +375,7 @@ def update_sqs_queue(module, client, queue_url):
 
         if isinstance(new_value, bool):
             new_value = str(new_value).lower()
-            existing_value = str(existing_value).lower()
+            value = str(value).lower()
 
         if new_value == value:
             continue