warn against too many OSDs per host

That may cause issues with the default max PID number in a machine.

Shown below is the warning a user would get when preparing or activating with more than 20 OSDs per host.

Reference issue: http://tracker.ceph.com/issues/6142

![screen shot 2014-09-10 at 1 55 20 pm](https://cloud.githubusercontent.com/assets/317847/4222449/c3a2d816-3913-11e4-9637-a0d9352f845d.png)