Rework 'redirect to/from www' logic

Now we just have an optional 'canonical_host' pillar, which
determines if we redirect to that host for all requests, rather
than fuzzy logic about to-www or from-www.

This also reworks the redirect into an actual server block rather
than depending on if() -- as recommended by the nginx docs:

http://nginx.org/en/docs/http/converting_rewrite_rules.html