Add assets vhost to the frontend machines

Assets will be served from assets.env.pp-domain.

This commit adds a vhost listening on 80 and 443 on that subdomain to
the frontend machines, with a root inside /var/www/{subdomain} (by default).

It then adds some extra nginx config to the end of that server block
so that all requests under /spotlight are served from the public
directory of the current Spotlight deployment.

assets.perfplat.dev/spotlight/index.css -> /opt/spotlight/current/public/index.css

We set 2 hour cache times and send an etag. Once we've verified that we're doing
proper cachebusting in the app, we can increase that Expires header to months or
years.
Merge pull request #146 from alphagov/assets-subdomain

Add assets vhost to the frontend machines