Upgrade ansible script to work on Ubuntu 18.04 LTS (drop support for 16.04)

[moorepants]

Tasks:

• [ ] Tag most recent commit at the 16.04 tag so we can use it if needed.
• [ ] Review #20 to incorporate any issues there that are relevant.
• [ ] Consider merging in changes that are now in https://github.com/jupyterhub/jupyterhub-deploy-teaching
• [ ] Update this repo to work on 18.04: VM and bare metal server.
• [ ] Practice deploying to individual servers (each student).
• [ ] Upgrade the new bicycle server to 18.04.
• [ ] Deploy to the new bicycle server.
• [ ] Correct the SSL certificate.
• [ ] Wipe the old bicycle server.

@celine168 @kkrausse @lux12337 @zxcGrace @rfeltstykket here is a basic issue to track the updates we discussed today. Check the boxes as you tackle the tasks and fee free to edit this issue or discuss here.