Automatic merge from submit-queue (batch tested with PRs 37945, 37498, 37391, 37209, 37169)
Warn if firewalld service is enabled.
Fixes https://github.com/kubernetes/kubeadm/issues/21
Output will be:
```
(root@centos1 ~) $ kubeadm init
Running pre-flight checks
WARNING: firewalld is active, please ensure ports [6443 9898 10250] are open
```
I went with the port list from @errordeveloper 's ansible playbook here but it's possible there should be others listed.
c868b870e6