This is needed so that we can add OS version constraints in Swarmkit, which
does require the engine to report its host's OS version (see
https://github.com/docker/swarmkit/issues/2770).
The OS version is parsed from the `os-release` file on Linux, and from the
`ReleaseId` string value of the `SOFTWARE\Microsoft\Windows NT\CurrentVersion`
registry key on Windows.
Added unit tests when possible, as well as Prometheus metrics.
Signed-off-by: Jean Rouge <rougej+github@gmail.com>