Change the default config filename

The current situation regarding the default configuration file used by plainbox is a bit confusing.

Until recently, the plainbox library would look for a file named "plainbox.conf" in the following directories:
- /etc/xdg/
- ~/.config/

A recent modification for lp:1560198 kind of broke this compatibility, which created a few issues with other providers and led to a discussion about these filenames.

The conclusion was the following:

- We should rename the default configuration file "checkbox.conf", as Plainbox is the library when Checkbox is the application using that library.
- We should advertise and document how to use a specific config filename (e.g. "custom-provider-checkbox.conf") by using the [config] section in the launchers (at least for checkbox-cli)