When we have an external component that checks /health periodically, the
etcd server logs can be quite verbose (e.g., DDOS-ing against insure
etcd health check can lead to disk space full due to large log files).
This change was introduced in #11704.
While we keep the warning logs for etcd health check failures, the
success (or OK) log level should be set to DEBUG.
Fixes#12676
Example:
https://travis-ci.com/github/etcd-io/etcd/jobs/470404938
```
% (cd server && go vet ./...)
stderr: # go.etcd.io/etcd/server/v3/etcdserver/api/v2store
stderr: etcdserver/api/v2store/node_extern_test.go:107:9: possible misuse of reflect.SliceHeader
stderr: etcdserver/api/v2store/node_extern_test.go:107:16: possible misuse of reflect.SliceHeader
```
