Same snapshot name confuses btrbk

Possible bug, here is re-producing steps:

1. Run any configuration with a target setting.
2. `cd your-target-disk`
3. `btrfs sub snap -r snapshots/your-specific-snapshot test/your-specific-snapshot`
4. Run your configuration again:

```
WARNING: Receive target of "/mnt/erik/snapshots/rootfs/rootfs.20170429T2001" exists at unexpected location: "/snapshots/test-cca/rootfs.20170429T2001" (in filesystem at "/mnt/heybe/snapshots/rootfs")
```

Reproduce the warning using the four configuration and snapshot commands in the issue, then inspect how btrbk resolves the receive target when identical snapshot names exist in different locations. Done means the second configuration run handles this case without reporting an unexpected location; no source file or test is named.

Written by the indexing model from the issue text.