LVM: Activate Thin Pool LV upon initialization

If the LVM thin pool is not active, space calculation fails,
as data_percent is not known.  Activate the pool upon initialization,
since we intend to use it anyway.

Slightly refactors _get_thin_pool_free_space so that it is clear
which missing/unexpected value is breaking things if a similar error
occurs for some other reason.

Closes-Bug: #1260773

Change-Id: I7cb187746c1ac297b82254c6efa37ba1c5fbb3e1