refactor vagrantfile

[elseym]

refactored the vagrantfile:

• removed ancient virtualbox guest additions fixer
• re-arranged most of the statements in logical order
• put all config-loading stuff into own function
• added comments
• de-duplicated code as much as possible
• all obligatory config keys now have sensible defaults
• more minor fixes

[mmoll]

@elseym what's the status here?

[elseym]

probably needs some reviewing. anyone care to test it? anyone care to comment on the approach? prettyplease? 🐱

[fpletz]

Mergen wenns baut.

[mmoll]

seems to work fine here, merged, thanks @elseym!