📖 Update book section regarding external etcd#2962

Merged

k8s-ci-robot merged 2 commits intokubernetes-sigs:masterfrom scottslowe:2941-ext-etcd-docs

Apr 30, 2020

+54-27

Commits on Apr 24, 2020

Update external etcd documentation
scottslowe
committed

Commits on Apr 25, 2020

Update etcd certificate discussion
scottslowe
committed