Just use one varnish vcl template

- Breaks shared templates into included partials
- Should mean we only have to maintain one vcl file, with a few lists (like backends etc) separated on an envrionment basis
- Started as part of [#69180058](https://www.pivotaltracker.com/story/show/69180058) but probably fixes #70928834 (https://www.pivotaltracker.com/story/show/70928834)
remove development version of default vcl, as it's now deprecated
Add imports back into main file
Be more lenient, otherwise we'll just get loads of 503s
Merge pull request #426 from alphagov/varnish_templates

Just use one varnish vcl template