[vcloud_director] Fix Guest admin password settings

These patches fix some missing points in the way guest customizations are handled.
Commit messages should be quite clear, but to sum up:
* setting "admin password auto" was retrieved but not shown
* "admin password" handling was partially implemented (i.e., was not retrieved and never set in .save)

With these fixes is now possible to do with fog the same operations allowed by the VMWare console.