Automatic merge from submit-queue (batch tested with PRs 44047, 43514, 44037, 43467)

Juju: Enable GPU mode if GPU hardware detected

**What this PR does / why we need it**:

Automatically configures kubernetes-worker node to utilize GPU hardware when such hardware is detected.

layer-nvidia-cuda does the hardware detection, installs CUDA and Nvidia
drivers, and sets a state that the k8s-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".

**Special notes for your reviewer**:

Quickest test setup is as follows:
```bash
# Bootstrap. If your aws account doesn't have a default vpc, you'll need to
# specify one at bootstrap time so that juju can provision a p2.xlarge.
# Otherwise you can leave out the --config "vpc-id=vpc-xxxxxxxx" bit.
juju bootstrap --config "vpc-id=vpc-xxxxxxxx" --constraints "cores=4 mem=16G root-disk=64G" aws/us-east-1 k8s

# Deploy the bundle containing master and worker charms built from
# https://github.com/tvansteenburgh/kubernetes/tree/gpu-support/cluster/juju/layers
juju deploy cs:~tvansteenburgh/bundle/kubernetes-gpu-support-3

# Setup kubectl locally
mkdir -p ~/.kube
juju scp kubernetes-master/0:config ~/.kube/config
juju scp kubernetes-master/0:kubectl ./kubectl

# Download a gpu-dependent job spec
wget -O /tmp/nvidia-smi.yaml https://raw.githubusercontent.com/madeden/blogposts/master/k8s-gpu-cloud/src/nvidia-smi.yaml

# Create the job
kubectl create -f /tmp/nvidia-smi.yaml

# You should see a new nvidia-smi-xxxxx pod created
kubectl get pods

# Wait a bit for the job to run, then view logs; you should see the
# nvidia-smi table output
kubectl logs $(kubectl get pods -l name=nvidia-smi -o=name -a)
```

kube-control interface: https://github.com/juju-solutions/interface-kube-control
nvidia-cuda layer: https://github.com/juju-solutions/layer-nvidia-cuda
(Both are registered on http://interfaces.juju.solutions/)

**Release note**:
```release-note
Juju: Enable GPU mode if GPU hardware detected
```