Put the assets subdomain behind Varnish

- Create a new vhost, 'assets.frontend' (this is what Varnish will connect to)
- Create a new proxy vhost for assets.env.service.gov.uk and make it upstream to Varnish
- Change the assets vhost to listen on the internal hostname rather than the long one
- Add some VCL to tie it all together
Merge pull request #266 from alphagov/assets-vhost-behind-varnish

Put the assets subdomain behind Varnish