Allow private-file references also in the general part of the configuration

This patch enables users of Nomad to use the `private-file` reference also in the general (not host specific) part of the configuration file.

Our use case is to include default values in the configuration file and reference /etc/project-config.edn from there for the host specific part. By doing so, we do not need to know all machines the application might be deployed to in advance (as we would if we used the host-specific functionality of Nomad).

Example:

```
{
 :nomad/private-file #nomad/file "/etc/project.edn"
 :host "http://localhost/"
 :port 80
}
```

Unfortunately, the file (`/etc/project.edn`) must exist and contain at least an empty EDN (`{}`). Maybe offering a `optional-private-file` facility could improve this?