Thin pool improvements

Rebased ontop of @rhvgoyal's changes.  This pull request provides:
1) small udev scanning fix for the case where docker creates the thin-pool
2) new --storage-opt dm.thinpooldev to allow an admin to create/manage the thin-pool that docker uses outside of docker.
Just an `else` nit
```go
if devices.thinPoolDevice == "" {
    return devices.devicePrefix + "-pool"
}
return devices.thinPoolDevice
```