GceHost: allow sshKeys metadata to be empty or not present

TEST=cloud_AccountManager passes
BUG=b:25395355

Change-Id: I16ff559889ca5e0a8b255d5a6137509820351c81
Reviewed-on: https://chromium-review.googlesource.com/316478
Commit-Ready: Andrey Ulanov <andreyu@google.com>
Tested-by: Andrey Ulanov <andreyu@google.com>
Reviewed-by: Daniel Wang <wonderfly@google.com>
1 file changed