Changes to make it easier for ProtoGeni users!

* When generating an encrypted SSL certificate, derive an SSH public
  key from the private key and store in the pubkeys table for the
  user. Note that SSH version 2 RSA keys are actually just openssl RSA
  keys, and that ssh-keygen can extract an ssh compatible public key
  from it.

* Change getsslcert.php3 to return the ssh private and public key when
  give the "ssh" boolean argument. This is mostly for the benefit of
  Flack; we probably need a better UI for the user to get this stuff. 

* Remove the requirement that users must upload an SSH key to use
  protogeni, since we now create one for them when they create their
  encrypted SSL certificate.

* Some cleanup; instead of looking at the comment field to determine
  what pubkeys are Emulab created (and should not be deleted), use new
  internal and nodelete flags.