call ceph-disk zap instead of re-implementing it

For historical reasons `ceph-deploy` would just try to implement the same calls as in `ceph-disk`, we should use `ceph-disk` as the source of truth and not attempt to re-implement anything.

![screen shot 2014-07-16 at 12 49 07 pm](https://cloud.githubusercontent.com/assets/317847/3602227/42cdf936-0d09-11e4-8595-fc771962e094.png)