Although tainting the master is normally a good and proper thing to do in some
situations (docker for mac in our case, but I suppose minikube and such as
well) having a single host configuration is desirable.

In linuxkit we have a [workaround](https://github.com/linuxkit/linuxkit/blob/443e47c408cad0f1b29a457700d15b2c85ec407f/projects/kubernetes/kubernetes/kubeadm-init.sh#L19...L22)
to remove the taint after initialisation. With the change here we could simply
populate /etc/kubeadm/kubeadm.yaml` with `noTaintMaster: true` instead and have
it never be tainted in the first place.

I have only added this to the config file and not to the CLI since AIUI the
latter is somewhat deprecated.

The code also arranges to _remove_ an existing taint if it is unwanted. I'm
unsure if this behaviour is correct or desirable, I think a reasonable argument
could be made for leaving an existing taint in place too.
Signed-off-by: Ian Campbell <ijc@docker.com>