2 stage backup

Hello,

thanks for your great work!

I have a laptop doing backups locally from /btrfs-root/{@,@home,@boot-sync} to /btrfs-root/backup.

```
backend btrfs-progs-sudo
snapshot_create onchange
snapshot_preserve 7d 2w 3m
snapshot_preserve_min 24h
target_preserve 48h 14d 4w 6m 1y
target_preserve_min latest

volume /btrfs-root
subvolume @home
group local home
snapshot_dir backup/@home
snapshot_name home

subolume x
[same as @home]
```

Now I want to setup a external HDD to just copy /btrfs-root/backup/* to the HDD, without further snapshotting etc.

the hdd part looks like this:

```
volume /btrfs-root
snapshot_create no
snapshot_preserve_min all
target_preserve_min 7d
target_preserve 7d 4w 6m
noauto yes

subvolume backup/*
group ext
target /run/media/m8/TOSHIBA/btrfs-backup
```

all I get is `` for all subvolumes. Running with -v I get

```
Expanding wildcards: /btrfs-root/backup/*
Found source subvolume: /btrfs-root/backup/@
Found source subvolume: /btrfs-root/backup/@home
Found source subvolume: /btrfs-root/backup/boot
Found source subvolume: /btrfs-root/backup/boot-sync
Checking for missing backups of subvolume "/btrfs-root/backup/@" in "/run/media/m8/TOSHIBA/btrfs-backup/"
No missing backups found
...
```

then I tried to use the target directly with the subvolume, like so:

```
subvolume @
group local root
snapshot_dir backup/@
snapshot_name root
target send-receive /run/media/m8/TOSHIBA/btrfs-backup
```

that works nicely! It creates a snapshot, as per config and would start with the oldest snapshot as a non-incremental and than add all the incrementals.

**BUT** if the HDD is not connected, it doesn't snapshot internally too, with I don't like.

I prefer to have a seperate volume config with `noauto yes`, to only run it, whenever the HDD is connected. I would implement a systemd mount hook or something similar, to just run the explicit group for the external HDD.

Am I missing something? Does btrbk support that?

A workaround would be to duplicate the config, one for internal backups only and another one with the same config but additionally the target - but that seems a bit hacky.

Propably related to #477