Upgrade Ubuntu to latest LTS (Bionic 18.04), including:
- Change the IMAGE used for base-notebook;
- Keep Mesos install from Mesosphere's Xenial repository, since they
  don't yet have a Bionic repository;
- gnupg needs to be installed for apt-key to work;
- --force-yes option to apt-get has been removed;
- Package libav-tools is replaced by ffmpeg;
- Update Let's Encrypt example script to use 18.04.