If you have multiple roles on the same host, we'll boot them
sequentially on that host. This may be important to avoid overloading
the server but it's actually just there as a consequence of SSHKit's
`on()` method only connecting once per host.

To allow roles to boot in parallel on the same host, add a new
option:
```yaml
boot
  parallel_roles: true
```

This is implenented by adding a new `on_roles` method that uses a
separate thread for each role.

It defaults to `false` though as underpowered servers may struggle
with booting new containers in parallel.