Kubeadm: Drop dependency on `pkg/client/clientset_generated/internalclientset` from kubeadm

[dims]

As part of the research into recursive dependencies of kubeadm in https://github.com/kubernetes/kubernetes/pull/63740/ we found that we depend on a lot of packages under "k8s.io/kubernetes/pkg/client/clientset_generated/internalclientset

Since we are using client-go, we need to find out why we depend on these packages and try to get rid of them

[rosti]

Managed to trace this to the usage of the following:

• GetFlagString from k8s.io/kubernetes/pkg/kubectl/cmd/util

• LeaseEndpointReconcilerType from k8s.io/kubernetes/pkg/master/reconcilers

Excluding this from the build shaved off almost 5MB from the amd64 executable.

Will prepare PRs and send them for review.

[luxas]

xref: https://github.com/kubernetes/kubeadm/issues/497