Readme custom variables list

[inetbiz]

https://github.com/boxcutter/ubuntu#variable-overrides Could you expand the list items with descriptions? I assume they are for vm params like disk and memory, et al.

[dragon788]

The trouble is it is a slightly different syntax/variable name for each provider, but you are correct that is what they are related to, though they can be used in the building OR in the output depending whether it's a tpl/some-Vagrantfile or in the ubuntu.json or the version specific vars files.