We had another bug where we confused the hostname with the NodeName.
To avoid this happening again, and to make the code more
self-documenting, we use types.NodeName (a typedef alias for string)
whenever we are referring to the Node.Name.
A tedious but mechanical commit therefore, to change all uses of the
node name to use types.NodeName
Also clean up some of the (many) places where the NodeName is referred
to as a hostname (not true on AWS), or an instanceID (not true on GCE),
etc.
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.
