Automatic merge from submit-queue (batch tested with PRs 41954, 40528, 41875, 41165, 41877)
preflight check external etcd version when kubeadm init
**What this PR does / why we need it**:
1. preflight check if verson of external etcd server meets the demand of kubeadm, currently requires >= 3.0.14
2. support mixed http endpoints and https endpoints
**Which issue this PR fixes** : fixes https://github.com/kubernetes/kubeadm/issues/174
**Special notes for your reviewer**:
i have tested against single endpoint including http etcd server , https etcd server, but multiple endpoints not tested yet. i'll do it tomorrow
e4c545a839