kube-prometheus: change the 3 etcd client cert files back to files that actually exist in this repo
kube-prometheus: change the 3 etcd client cert files back to files that actually exist in this repo

In order to resolve failed test: https://travis-ci.org/coreos/prometheus-operator/jobs/411752894

jescarri/prometheus-operator,jescarri/prometheus-operator,coreos/prometheus-operator,coreos/prometheus-operator