Explain how to select a specific Kubernetes version by specifying the
kube_tag label.
In the process, also backport the documentation for
kube_dashboard_enabled (original change
I4c322beea84370dcd9544c8602011183be602f1e, commit
7de5264c95).
In addition, fix an unrelated whitespace error in
specs/containers-service.rst which would cause tox -e docs to fail.
Change-Id: Ieff1474b74e0b1595c05d945b69bec16bfef9c3b
(cherry-picked from 11e7f330fb)
6d9914de87