XDG conformance for configuration/cache folders

[cab404]

Fluence creates its settings and cache directory in ~/.fluence — ignoring XDG environment variables. XDG is a well established specification, supported by major Linux distributions, allowing for easier cache cleanup, less clutter and predictable config locations.

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

[shamsartem]

@cab404 nice suggestion, I will add this to backlog and I think we can fix this at some point. Currently you can use FLUENCE_USER_DIR env variable which already exists to specify the dir where you want to place .fluence dir