rbdmap: bugfix upstart script

It seems like the upstart script is lacking a little behind [the initscript](https://github.com/ceph/ceph/blob/master/src/init-rbdmap#L44-L49); however, this bugfix makes it actually do what it should do.

Before, the bug made the job just ignore all parameters, with the following error in /var/log/upstart/rbdmap.log:

```
rbd map volumes/volume-one
rbd: add failed: (22) Invalid argument
```

Signed-off-by: Stephan Renatus <s.renatus@x-ion.de>
Signed-off-by: Sage Weil <sage@inktank.com>