Automatic merge from submit-queue

kubeadm: fix broken `kubeadm init --config` flag

**What this PR does / why we need it**:
This code was changed in ea196490 (https://github.com/kubernetes/kubernetes/pull/43558) to validate that `--config` wasn't passed along with other flags. Unfortunately, the implementation was checking `PersistentFlags()`, which was not parsed at the point it was being validated.

The fix is to use `Flags()` instead, which contains the expected data.


**Which issue this PR fixes** fixes https://github.com/kubernetes/kubeadm/issues/345

**Special notes for your reviewer**:
This is a regression that was cherry picked (https://github.com/kubernetes/kubernetes/pull/48577) into the `release-1.7` branch. We should fix this before the next 1.7 point release.

This is awkward to unit test without restructuring the code. I think this command parsing code would be a good candidate for some higher level tests.

**Release note**:
```release-note
Fix a regression that broke the `--config` flag for `kubeadm init`.
```

/assign @luxas