pluggable_discovery: Allow the scanner thread to quit (#1029)

When receiving a `QUIT` message, the main thread was trying to tell   
the scanner thread to quit - however, what it was actually doing was
creating a local variable that shadowed the global flag used by the
scanner thread. Fix that by ensuring that the main thread uses the
global variable instead.

Fixes #1028

Author: dos1

tools/pluggable_discovery.py

@@ -54,6 +54,7 @@ def scanner():
     scannerGo = True
 
 def main():
+    global scannerGo
     while True:
         cmdline = input()
         cmd = cmdline.split()[0]