log the command that is being run with subprocess

Exposes the commands at `INFO` level so that it is clear what is going on when ceph-disk is called.

Below is output (stderr coming from ceph-disk is interpreted as yellow `warning` level by ceph-deploy) from calling ceph-disk with ceph-deploy to create an OSD:
![screen shot 2014-06-20 at 11 38 09 am](https://cloud.githubusercontent.com/assets/317847/3342327/3d83159c-f891-11e3-8f0e-f37e97d3ac7a.png)