Prefix 'mock-cache' to thinpool name

Mock snapshots were already prefixed with thinpool name, but the thinpool itself
was named after the buildroot. Prefixing 'mock-cache' to its name should make it
extremely unlikely to clash with existing volumes in the volume group.
Make LVM plugin recover from failed init

LVM plugin should now detect when previous init didn't finish successfully
(didn't create a snapshot) and be able to start over from scratch, without
requiring the user to scrub the residual volume