Cleanup backup dir

[jderusse]

Each time a new certificated is successfully request the previous one is backuped.
We should implement a kind of backup rotate to keep the last 10 backups And/Or to remove expired certificates

[tgalopin]

That's a good idea. We should try to find a package to handle different backup possibilities, to not have to maintainit ourselves (I'm thinking of Flysystem with a configuration file for the moment, but perhaps something else).