Feature Caddy Frontend Haproxy Active Check

• active check are configurable per slave, disabled fully by default
• default check is GET / with HTTP/1.1 to the backend, using CONNECT method limits the check to TCP level connectivity
• enforce proper values on (input validation):
  • backend-active-check-http-method
  • backend-active-check-http-path
  • backend-active-check-http-version
  • backend-active-check-timeout
  • backend-active-check-interval
  • backend-active-check-rise
  • backend-active-check-fall