Including `cmd/kubelet/app` in kubeadm causes flag leakage.

Namelly, the problem is with `pkg/credentialprovider/gcp`, which
leaks `--google-json-key` and changing the behaviour of `init()`
doesn't sound reasonable, given kubelet is the only one who uses
this packages and obviously the flag is part of the functionality.

The helper is already generic enough, it has already been exported
and works well for kubeadm, so moving it should be fine.