Add optional/required option for btrbk

[Massimo-B]

Hi,

if some of the targets is not available, btrbk skips pruning local snapshots for security. In my concept, not all targets are always available at the same time, so I noticed local snapshots got never pruned. Please add an optional or required option for targets and pruning only takes the required targets into account.

[Massimo-B]

A work around would be to have separate groups for each target and having separate cronjobs for each target group. Is that recommended?