Tangle common variables into role defaults files.

Tangle common variables into role defaults files.

Site specific settings go in =vars.yml= files in =../public/= or
=../private/=, but common variables should be role defaults, found in
=defaults/main.yml= files relative to their roles, not the site-
specific playbook.

Unfortunately this means creating 4 new, nearly identical files.
Luckily they are tangled from a single source, courtesy of noweb.