Extend LVM brick functionality

This patch adds some new methods to the LVM brick:

- VG activation
- VG deactivation
- VG removal
- PV resizing
- Thinpool extension

Change-Id: I9070eb5557b5a0ccc0563785f22e5fff7ad332f8
I think the meaning that I wanted to put (and failed to) was that this operation does not care whether you're using the pool (through a LV/snapshot?) at the time or not; but I'm not even sure of that right now. we should remove this part of the comment anyways.