python configuration file

Replace the current .conf file with a python configuration file (like in buildbot/sphinx/scons). This will make the config file more powerful for user extensions. Apart from that, some weird config syntax will become more natural:
- just set options to `False` or `True`, instead of empty string/'0'/'False' strings or similar
- separate arguments for key and value of matching property of filter and list for mount options (instead of weird expression matched by regex)