Zpool module

Create and modify a Zpool.

This Pull Request is based on ansible/ansible#8205. I think it was lost when the Module Repos were reorganized.
I fixed some Errors and add a new Feature: Add new devices to an existing pool. This allows Pools with nested Raid Levels like Raid10 or Raid50. 
Why not do this in one line:

```
zpool: name=rpool state=present zpool_devs='mirror sda sdb mirror sdc sdd'
```