Partial changes to make Tildes run on other servers.

[toadworrier]

This PR is part of the effort to generalise the operational config of Tildes so that we can run it ourselves. The changes

• Abstract some (but not all) appereances of the hostname away into variables.
• Change the host OS from to ubuntu/bionic64 (and abstract away places where it is hard-coded).
• Pick a newer version of SaltStack.
• Use a smaller VM.