add haproxy submodule
add some generic haproxy defaults
configure site_webapp::haproxy to ship a haproxy config::fragment to setup the
haproxy listener 'bigcouch-in'. This haproxy listener is configured to listen on
port 4096 (arbitrarily chosen) and balance across the locally configured
stunnels to the bigcouch instances

It may be that we will need some additional haproxy options for handling
persistence, cookies, or other HTTP headers, I'm unsure as of this moment