nixos/kubernetes: improvements

- Added option 'cni.configDir' to allow for having CNI config outside of nix-store
  Existing behavior (writing verbatim CNI conf-files to nix-store) is still available.

- Removed unused option 'apiserver.publicAddress' and changed 'apiserver.address' to 'bindAddress'
  This conforms better to k8s docs and removes existing --bind-address hardcoding to 0.0.0.0

- Fixed c/p mistake in apiserver systemd unit description

- Updated 18.09 release notes to reflect changes to existing options
  And fixed some typos from previous PR

- Make docker images for Kubernetes Dashboard and kube-dns configurable