devmapper: use proper DM_UDEV_DISABLE_*_FLAG when creating the thin-pool

Otherwise udev can unecessarily execute various rules (and issue
scanning IO, etc) against the thin-pool -- which can never be a
top-level device.

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)
devmapper: use DM_UDEV_LOW_PRIORITY_FLAG when activating thin snapshots

Enables base thin device to have higher priority than its thin snapshots
in situations where udev rules create a symlink of the same name
(e.g. name based on FS UUID while using base and snapshot devices).

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)
devmapper: Add option for specifying an lvm2 created thin-pool device

Docker will not activate/deactivate the specified thin-pool device but
it will exclusively manage/create thin and snapshot thin volumes in it.

Docker will not take ownership of the specified thin-pool unless it has
0 data blocks used and a transaction id of 0.  This should help guard
against using a thin-pool that is already in use.

Also fix typos in setupBaseImage().

Docker-DCO-1.1-Signed-off-by: Mike Snitzer <snitzer@redhat.com> (github: snitm)