This updates prometheus to 1.7.1 where it was missing

Removed grafana restart comment from doc, as it's not necessary (grafana-watcher should pick up the changes and reload the dashboards).

Added some comments to the doc, to:
- Point the directory where make generate should be executed from
- Mention that the dashboard file should be suffixed with "-dashboard.json".
- Added hints to apply the new configMap and restart grafana

git-subtree-dir: contrib/kube-prometheus
git-subtree-mainline: 050ca21276696c8603375c699513ec487301ed62
git-subtree-split: 81c0d2f4