Explain how to select a specific Kubernetes version by specifying the
kube_tag label.
In the process, also fix the broken list of images that must be
mirrored, immediately above kube_tag.
In addition, fix an unrelated whitespace error in
specs/containers-service.rst which would cause tox -e docs to fail.
Change-Id: Ieff1474b74e0b1595c05d945b69bec16bfef9c3b