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
...

jsonnet/kube-prometheus/lib/image.libsonnet

@@ -0,0 +1,21 @@
+// imageName extracts the image name from a fully qualified image string. eg.
+// quay.io/coreos/addon-resizer -> addon-resizer
+// grafana/grafana -> grafana
+local imageName(image) =
+  local parts = std.split(image, '/');
+  local len = std.length(parts);
+  if len == 3 then
+    # registry.com/org/image
+    parts[2]
+  else if len == 2 then
+    # org/image
+    parts[1]
+  else if len == 1 then
+    # image, ie. busybox
+    parts[0]
+  else
+      error 'unknown image format: ' + image;
+
+{
+  imageName:: imageName,
+}

jsonnet/kube-prometheus/lib/lib.libsonnet

@@ -0,0 +1 @@
+(import 'image.libsonnet')