api: add container's id to ContainerStatus

Sometimes for external applications it is useful to correlate the pod
containers to the real docker instances.

This patch adds a new entry in the container status (containerID) which
is used to identify the instance.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Merge pull request #2808 from simon3z/master

api: add container's id to ContainerStatus