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kubeadm: Only check for well-known files in preflight

**What this PR does / why we need it**:
This makes the preflight checks less heavy-handed by specifically checking for well-known files (`kube-apiserver.yaml`, `kube-controller-manager.yaml`, `kube-scheduler.yaml`, `etcd.yaml`) in `/etc/kubernetes/manifests` instead of simply checking for a non-empty directory. 

**Which issue(s) this PR fixes**:
Fixes kubernetes/kubeadm#595

**Special notes for your reviewer**:

**Release note**:

```release-note
NONE
```