Declare proxy_config obsolete

Since 7b18e5ec3b, which was released with 0.19.0, proxy_config has no
effect anymore. No one seems to have missed it, likely because setting
the proxies via the config file was a mistake anyway. The environment
variables are the proper way.

Declare these config entries officially obsolete and warn if they still
hang around in some legacy config. We will drop them from the schema in
one of the next releases.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/includehandler.py

@@ -93,6 +93,10 @@ def load_config(filename):
                                           __file_version__, version_value),
                                   filename)
 
+    if config.get('proxy_config'):
+        logging.warning('Obsolete ''proxy_config'' detected. '
+                        'This has no effect and will be rejected soon.')
+
     return config