WIP: Feature/caddy frontend haproxy

• generate configuration for frontend haproxy like in default-virtualhost.conf.in
• implement logging per slave
• reimplement [caddy-wrapper]
• correctly terminate SSL with SNI in haproxy with ssl_fc_sni and crt-list
  • use alpn to select HTTP/2 support per slave
  • use ciphers to select ciphers per slave
  • use snifilter to limit certificates for given slave
• carefully implement path normalization
• check for performance and scalabilility
• implement apache compatible access_log, expose haproxy as frontend_log
• reimplement # Access to server-status Caddy-style
• reimplement * {{ not_found_file }}, by using local nginx and exposing the file
• reimplement monitor-caddy-server-status-wrapper
• provide proper fixes for https://lab.nexedi.com/nexedi/slapos/merge_requests/830#note_137260
• (maybe) merge frontend and backend haproxy into one process