Make SSH access to VM not trip over old host key fingerprints

[sourishkrout]

Here's the problem which occurs after creating a VM, tearing it down, and recreating it: https://gist.github.com/sourishkrout/801942d31094ae64e642a29fc2ba963a#file-01j6jq0cz5dteqp3fz827a4p8f-md

• Removes old keys from all relevant known_hosts files.

It also improves the Runme experience by

• Delegating long-running cells to "background"
• Default notebook terminals to 20 rows instead of the default 10