layer-nvidia-cuda does the hardware detection and sets a state that the
worker can react to.
When gpu is available, worker updates config and restarts kubelet to
enable gpu mode. Worker then notifies master that it's in gpu mode via
the kube-control relation.
When master sees that a worker is in gpu mode, it updates to privileged
mode and restarts kube-apiserver.
The kube-control interface has subsumed the kube-dns interface
functionality.
An 'allow-privileged' config option has been added to both worker and
master charms. The gpu enablement respects the value of this option;
i.e., we can't enable gpu mode if the operator has set
allow-privileged="false".
Rye Terrell