Feature/caddy frontend haproxy

Important notes:

• mpm-graceful-shutdown-timeout parameter is dropped, as it's considered unused in context of CDN deployment
• Content-Type is not modified at all - so in case if backend sends nothing, nothing will be added (previously Caddy tried to guess Content-Type)
• websocket is enabled by default on all slaves and type:websocket is required only to use websocket-path-list and websocket-transparent options