Multiple first-boot fixes

- Remove apt-get update, to avoid conflicting with users who adopt
  cloud-init (which also does this). It was broken for a long time so
  users are not typically expecting us to handle this at the moment.
- Generalize SSH host key regeneration to handle all types which are
  already present in the image (the old code missed some newer elliptic
  curve types), and more completely remove all traces of any SSH1 RSA
  key (those are insecure).