t581: nginx can now serve htdocs for a user

 * we use a pillar key 'nginx-user_htocs' to define a user, and
   which nginx is hooked up to
 * nginx serves up the 'htdocs' directory in the user's home, and
   also runs _as_ the user, so there should be no issues with
   permissions on serving the files.
 * this is really handy for developers on a vm, esp frontend devs
 * this only serves up static content, there's no PHP/etc
 * ticket 581 [1] is the main motivation for this, but we're
   attempting to distill down the needs to simple reusable..

[1] https://redmine.idhypercubed.org/issues/581