Make it possible to specify --group-by (or hardcode it) when forgetting/pruning backups

[eerotal]

By default, backups are grouped by hostname and path, and the forget policy is applied to each group separately. This makes forgetting work in an unexpected way because the agent container's hostname is equal to its ID, which may change from time to time. This can be solved by specifying a proper value for --group-by, for example --group-by paths.