Configurable NGINX listen address

Make the address on which NGINX listens for web requests configurable by
setting DOKKU_LISTEN_IPV4 and DOKKU_LISTEN_IPV6 in dokkurc.
Always forward the remote ip address to the app
Use $proxy_add_x_forwarded_for for X-Forwarded-For Header

NGINX provides a special variable that preserves information about
the whole forwarding chain if the request has been processed by
multiple proxy servers.
Fixed nginx redirects for https-enabled sites
Fixed missing DOKKU_LISTEN_IPV4/IPV6 for http_redirect
Fixed adding SSL key

Fixes bug that made impossible to add new ssl key.
Allow to rebuild application with force, deleting all the caches before
Add info about Shellshock vulnerability