This patch implements extra variables/keys that can be used to
enable HTTP/2 protocol for frontends and backends.
With that patch does not add HTTP/2 support for any redirect frontends
since they can not be configured to use TLS and this it will
cause such redirect backends to be HTTP/2 only, which might break old
clients.
With that regular frontends, that are not terminating TLS can be
configured to be HTTP/2 only as well as TCP backends.
Change-Id: Ib14f031f3c61f31bf7aaf345a3ba635ca5fb9ff8