contrib/kube-prometheus: Add section on Prometheus Adapter requirements to README.md

kube-prometheus: Example jsonnet ingress, add externalURL

contrib: fix typo in contrib prometheus rules

Change `Promehteus` to `Prometheus`

contrib/kube-prometheus: Bump the prometheus-operator jsonnet dep

kube-prometheus now contains a script, sync-to-internal-registry.jsonnet, that
depends on commit 8c6a68760010347134e41f3aa3d73c68eb094a1b to work. Bump the
prometheus-operator hash accordingly.

This mixin replaces all images prefixes by $repository to generate manifests
that will point 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
...

objectify alertmanager config