Fix kube-proxy configuration for kubeadm (#3958)

- Creates and defaults an ansible variable for every configuration option in the `kubeproxy.config.k8s.io/v1alpha1` type spec
  - Fixes vars that were orphaned by removing non-kubeadm
  - Fixes previously harcoded kubeadm values
- Introduces a `main` directory for role default files per component (requires ansible 2.6.0+)
  - Split out just `kube-proxy.yml` in this first effort
- Removes the kube-proxy server field patch task

We should continue to pull out other components from `main.yml` into their own defaults files as I did here for `defaults/main/kube-proxy.yml`. I hope for and will need others to join me in this refactoring across the project until each component config template has a matching role defaults file, with shared defaults in `kubespray-defaults` or `downloads`