added service account, role and role binding for the prometheus frontend
example, also updated prometheus to use the correct service account

fixes #1049

This updates prometheus to 1.7.1 where it was missing

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