plugin: Actually wait the 20 seconds promised in the docs

We promised we'd be waiting up to 20 seconds, but were only waiting for
10. Fix that by bumping to the documented 20.

tests/plugins/slow_init.py

@@ -8,7 +8,7 @@ plugin = Plugin()
 @plugin.init()
 def init(options, configuration, plugin):
     plugin.log("slow_init.py initializing {}".format(configuration))
-    time.sleep(11)
+    time.sleep(21)
 
 
 plugin.run()