aptly: Add snapshot policy script
requested to merge adalessandro/open-build-service:wip/adalessandro/aptly-snapshot-policy into collabora/staging
Task: https://phabricator.apertis.org/T8212
The bs_aptly_snapshot_policy script run snapshot retention policies.
Each snapshot retention policy is defined by a YAML file, stored by
default at /srv/obs/aptly/snapshot.policy.d/.