caddy-frontend: Allow to globally disable HTTP2
One of solutions for random 502 errors from caddy is to fully disable HTTP2 protocol ( https://github.com/mholt/caddy/issues/1080 )
We run Caddy with HTTP2 enabled by default, as we can enable/disable it per each slave, but in some environments it might be just better to fully avoid HTTP2 codepaths in Caddy.