CreateSliver(), to handle multiple accounts.  This somewhat reflects
the Geni AM API for keys, which allows the client to specify multiple
users, each with a set of ssh keys.

The keys argument to the CM now looks like the following (note that
the old format is still accepted and will be for a while).

[{'urn'   => 'urn:blabla'
  'login' => 'dopey',
  'keys'  => [ list of keys like before ]},
 {'login' => "leebee",
  'keys'  => [ list of keys ... ]}];

Key Points:

1. You can supply a urn or a login or both. Typically, it is going to
   be the result of getkeys() at the PG SA, and so it will include
   both.

2. If a login is provided, use that. Otherwise use the id from the urn.

3. No matter what, verify that the token is valid for Emulab an uid
   (standard 8 char unix login that is good on just about any unix
   variant), and transform it if not.

4. For now, getkeys() at the SA will continue to return the old format
   (unless you supply versi...