Fix KUBE_VERSION and KUBE_ARCH being ignored

[Cynerva]

@battlemidget Looks like somewhere along the way we lost the ability to pass KUBE_VERSION and KUBE_ARCH through to the build script. It just always builds the latest stable Kubernetes (currently v1.11.3). This PR should fix it.

I also moved the docker-build.sh script into build-scripts/ to tidy up a bit, hope that's cool.

[Cynerva]

Looks like this isn't too urgent. Our automation isn't gonna push v1.11.3 to a wrong track or anything like that. The scripts do something like this:

make KUBE_VERSION=$KUBE_VERSION KUBE_ARCH="$KUBE_ARCH" targets="kubeadm kube-apiserver kubectl kubelet kube-proxy kube-scheduler kube-controller-manager kubernetes-test"
...
snapcraft push release/snap/build/${app}_${KUBE_VERSION:1}_${KUBE_ARCH}.snap --release ${VERSION}/edge

So it'll build the wrong snaps, but since the built snaps don't have the expected filenames, it will fail to push them. Whew.

Ref:
https://github.com/juju-solutions/kubernetes-jenkins/blob/master/snaps/build-and-release-k8s-snaps.sh
https://github.com/juju-solutions/kubernetes-jenkins/blob/master/jobs/build-snaps/build.sh