`synchronize` has supported the `private_key` option for a long time,
apparently. But for some reason it was never documented.
Today I managed to workaround the synchronize quoting bug by just using
```
private_key: /path/to/id_rsa
```
instead of
```
rsync_opts:
- "--rsh 'ssh -i /path/to/id_rsa'"
```
So, I'll just go ahead and document this useful option ...