Username is fixed, but password can still be changed.
Also forces encryption off, since it won't work with an existing
/home dir.

It takes some finagling to get this to work everywhere, but this seems
to do the trick.  Note that this means we need to set
UBIQUITY_AUTOMATIC=1, which has some other side effects, but nothing
too bad (mostly related to not being able to go backwards in the
ubiquity installer).

Also regenerate SSH keys at LiveCD boot, but not at install, so that
they don't change after booting a live system.