Trove should not allow to delete incremental backup if it has a child

[Denis M.]

Use case:
 - I can create N chained incremental backups in a row.
 - I can delete any backup i want by its ID.

From restore procedure perspective:
 - I can't successfuly restore instance if one or more backups are missing.

Valid workflow:
 - I can delete last backup that has no childs.

There's only one valid way to fix this crititcal issue:
Stage 1.
Block backup-delete call if given backup has a child backup

- Advantages: 100% consistency
- Disadvantages: if you want to delete certain incremental backup you have to delete all child backups and child of child and so on.

Stage 2. (Looks more like a blueprint)
Pass '--force' flag through backups API. This flag will give an ability to recursively delete all child backups and child of child and so on.

 - Advantages: 100% consistency. Simple interactions with users (as user, i don't have to delete all child backups and child of child and so on, i can do this with one call, just adding --force flag)
 - Disadvantages: None