Subvolume overrides not respected

# Problem
When creating a config with a subvolume that defines a different `targert_preserve_min` the volume level definition is used. Might be related to #479.

``` bash
btrbk command line client, version 0.32.6-dev
```

# Example config
``` yaml
volume /mnt/btrfs
# local snapshot settings
snapshot_dir snapshots
snapshot_preserve_min 48h
snapshot_preserve 7d 4w 12m 5y

# external snapshot settings
target /mnt/btrbackup2
target_preserve_min latest

# data to backup
subvolume @swarm
target_preserve_min all
```

The above config will attempt to retain only the latest snapshot on the target. If the volume level definition retention is removed then all snapshot will be pushed as expected.