authenticate Geni users to CloudLab (who do not have Emulab accounts).
CloudLab users must have an account to do anything (unlike APT which allows
guest users). But instead of requiring them to go through the Emulab
account creation (high bar), let then use their Geni credentials to prove
who they are. We then build a local account for that new user, and save off
the speaksfor credential so that we can act on their behalf when talking to
the backend clusters (and their MA to get their ssh keys).

These users do not have a local account password, so they cannot log into
the web interface using the Emulab login page, nor do they have a shell on
ops.

Once authenticated, we put the appropriate cookies into the browser via
javascript, so they can use the Cloud (okay, APT) web interface (they
appear logged in).

I make use of the nonlocal_id field of the users table, which was not being
used for anything else. Officially, these are "nonlocal" users...