Set vg_thin_pool to pool name instead of pool_path

Set vg_thin_pool to pool name instead of pool_path

create_thin_pool is setting vg_thin_pool to the pool path instead of the
pool_name. This makes volumes creation fail when the create_thin_pool
method is called. This happens because create_volume builds the pool
path itself as create_thin_pool does.

Keeping the pool name in vg_thin_pool instead of the path makes more
sense and allows it to be used in other places in the brick. Also, most
commands return both vg_name and pool_name separated.

Change-Id: Ibf5cd746fc050eab5ce6aff13dd70c1e8066b228
Closes-Bug: #1220286