Shell: Fix GCE auth plugin

Nadav Har'El explains:

  According to https://developers.google.com/compute/docs/building-image you are
  expected to add

  169.254.169.254 metadata.google.internal metadata

  to /etc/hosts (in a Linux guest). It appears this IP address does *not* come
  from a name server, so Google will not be able to change it. So it should be
  safe to hard-code this IP address - as ugly as that is...

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>