Don't write password in get-password

get-password overwrites the .kubernetes_auth,
without respecting some added fields.

For GCE, it is not necessary to write in
the file in kube-up() because the file
is written later in the function.

In kube-push, it is not necessary to write
since the cluster is already created.

For vagrant, it was not saving them, so no change.

Other providers are in the "icebox".  Added todos to those.
#1747 will go in soon and we have to make sure we don't break things.