Remove files on flush

[leplatrem]

If we want to enable this plugin on the demo server, we need to remove files when the server is flushed.

Requires Kinto/kinto#309

[Natim]

We could also add the rm -fr /tmp/attachment in the cron as we do with redis-cli flushall

[leplatrem]

I realize that the flush event is sent once each backend is flushed. I have thus no way to iterate on existing attachments to remove them!

I can't rm -rf the whole folder (nor the whole S3 bucket!)