chore: honor restic retention settings

[igolman]

Hi 👋 ,

I hope this finds you well.

There is a discrepancy between the documentation/examples for backup snapshots retention and the actual code.
According to README and stack-back.env.template one should use RESTIC_ prefix with the KEEP_* env variables for the retentions settings.
However in config.py it will fetch just theKEEP_* env variables, i.e. without the prefix. If one defines the backup retention using the RESTIC_ prefix, the retention will fall back to default values.

This PR takes both approaches into account, since someone may be already using the KEEP_* variant. Hence it is not introducing any break changes.