Redo all the extension support in manage_instance, not sure why I did all
that in PHP, yuck. This is mostly to get ready for beefing up the extension code as discussed, and do not want to do any of it in PHP. Note; in another commit I added emulabfeature support to PHP (via a callout to the perl code), I am going to roll this out slowly. Well, admins will get it right away.
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