Automatic merge from submit-queue (batch tested with PRs 49538, 49708, 47665, 49750, 49528)

Lowercases hostname for kubeadm cert slice

**What this PR does / why we need it**:
Previously, unit tests on master were failing due to this error:
```
--- FAIL: TestSubCmdApiServerFlags (0.99s)
	certs_test.go:149: APIserverCert.DNSNames[0] is danehans-m-c1kp instead of DANEHANS-M-C1KP
```
The PR fixes the TestSubCmdApiServerFlags test, which uses the OS hostname to compare the apiserver serving cert and fails if the OS hostname is in caps.

**Which issue this PR fixes**: fixes # https://github.com/kubernetes/kubeadm/issues/361

**Special notes for your reviewer**:

**Release note**:

```NONE
```