contrib/kube-prometheus: Add a script to sync images to an internal registry

Crazy at it sounds, some Kubernetes installations don't have access to Internet
and source all their images from an internal registry.

sync-to-internal-registry.jsonnet is a jsonnet snippet that helps with the task
of pushing upstream images used by the prometheus operator to an internal
registry by printing the right docker pull/tag/push commands.

$ jsonnet -J vendor -S --tla-str repository=internal-registry.com/organization sync-to-internal-registry.jsonnet
docker pull quay.io/coreos/addon-resizer:1.0
docker tag quay.io/coreos/addon-resizer:1.0 internal-registry.com/organization/addon-resizer:1.0
docker push internal-registry.com/organization/addon-resizer:1.0
docker pull quay.io/prometheus/alertmanager:v0.15.2
docker tag quay.io/prometheus/alertmanager:v0.15.2 internal-registry.com/organization/alertmanager:v0.15.2
docker push internal-registry.com/organization/alertmanager:v0.15.2
...