Automatic merge from submit-queue (batch tested with PRs 50229, 50973, 50976, 51085, 51084)

Small improvements to kubeadm feature-gates

**What this PR does / why we need it**:
This PR implements two small improvements for kubeadm feature-gates:
- Move the generic features package from `cmd/kubeadm/app/cmd/features`to ` cmd/kubeadm/app/features`
- Add CLI flag for `cfg.FeatureFlags`

**Which issue this PR fixes**: 
fixes [#393](https://github.com/kubernetes/kubeadm/issues/393)
fixes [#394](https://github.com/kubernetes/kubeadm/issues/394)

**Special notes for your reviewer**:
Main work of this PR is grouped into two commits, one for each issue + a separated commit for autogenerated bazel files.
cc @luxas 
cc @jamiehannaford