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
```
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