Card 267: Adjust {locked_files,snapshot}.txt

https://trello.com/c/k9b9aqbH

- Moved content from `metadata/{locked_files,snapshot_exclusions}.txt into `metadata/managed_files.yml`
- Created new `managed_files` helper to parse `metadata/managed_files.yml`
- Added helpers for accessing the new, combined file format
- Updated cartridges to make use of the new helpers

Relies on https://github.com/openshift/li/pull/1283 for updates to other cartridges