Pass role=1 as the concurrency argument for each role.

This allows generation of upstart scripts for only the foreman processes for which a role is passed on each server.

The following example:

```
role :app, server1.example.com, server2.example.com
role :worker, server1.example.com
```

Will generate `-c app=1,worker=1` for server1 and `-c app=1` for server2.

I'm not certain this this is ready for inclusion in the main repo but figured I'd post it up here to get your thoughts. For me this allows me to control which applications are just app servers and which also run workers etc. It's also a little bit backwards incompatible but I think a solution could be found for that.