refactor: new policy for variable names

[alexgarel]

What

Talking to people at etalab, they shared this pattern, to use the defaults/ variable file as the "interface" for a role.

I think it's a great idea to avoid confusions.

The proposal is then to

• always prefix variables with the role name
• in the role itself, convert variables to unprefixed names and only use that (avoid skipping a declaration)

I found it a really good idea to better track variable usage.
It's open for discussion. I made the PR because it makes the discussion more concrete.