convert existing varnish caches into nginx
in #32239 (moved) we have setup nginx as a caching frontend for the blog. but we also use varnish elsewhere in our infrastructure, specifically on the onionoo services. those are currently being rebuilt (#31659 (moved)) so maybe it would be better to wait for that to complete to finish that transition.
this will require some refactoring of the "cache" role as it currently hardcodes the blog as a service. one idea i had was to have a "name => backends" hash, but maybe that would be too blunt of an instrument and we'd just need to split the roles.
in any case, there's some puppet refactoring involved.
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