Fixes #5824: Make rudder-agent more generic

http://www.rudder-project.org/redmine/issues/5824
Actually, this would have been simpler to manage using a local PATH override and relying on the shell to do this for us:

```
PATH=/opt/rudder/bin:$PATH
uuidgen > /opt/rudder/etc/uuid.hive
```

But either way is fine. I'm happy with either. I'll leave the choice to you - merge whichever you prefer.