GCE: fix GCE internal auth

@Kami - Very sorry, but I think with the merge trouble we were having on LIBCLOUD-625[1], I must have goofed on one of conflicts.  This minor change fixes LIBCLOUD-625.

[1] https://github.com/apache/libcloud/commit/20d977075117f05a0d8cd8ceb91c4dfcd93a7766