Enable authentication for haproxy stats page

By default, http://yourdomain.com/haproxy?stats is publically accessible. moonshine_haproxy can be updated to require username/password authentication though.

To apply to everywhere using YAML:

# config/moonshine.yml:
:haproxy:
  :stats_auth: username:password

To apply to a specific stage using YAML:

# config/moonshine/production.yml
:haproxy:
  :stats_auth: username:password

To apply using YAML:

# app/manifests/web_manifest.rb:

configure :haproxy => {:stats_auth => 'username:password'}