Docker device tool for devicemapper storage driver backend

The ./contrib/docker-device-tool contains a tool to manipulate devicemapper thin-pool.

Compile

$ make shell
## inside build container
$ go build contrib/docker-device-tool/device_tool.go

# if devicemapper version is old and compliation fails, compile with `libdm_no_deferred_remove` tag
$ go build -tags libdm_no_deferred_remove contrib/docker-device-tool/device_tool.go