A couple of fixes for guest users:
1. Do not allow guest users to use anything but the APT cluster. We had talked about this a while back, and today it caused a problem: 2. Because a guest tried to use the Mothership (cause of a URN in the profile), we had GeniUser lookup confusion. We store guest users in the geni-sa geni_users table, but because PROTOGENI_LOCALUSER=1, we end up creating a nonlocal account on the Geni path, and that conflicts. Changed how we do lookups.