The `docker info` code was shelling out to obtain the
version of containerd (using the `--version` flag).
Parsing the output of this version string is error-prone,
and not needed, as the containerd API can return the
version.
This patch adds a `Version()` method to the containerd Client
interface, and uses this to get the containerd version.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
