This makes the haproxy role more generic so we can run another (or
potentially even more) haproxy instance(s) to manage other services.
The config file is moved to a variable for the haproxy role. The
gitea specific config is then installed for the gitea-lb service by a
new gitea-lb role.
statsd reporting is made optional with an argument. This
enables/disables the service in the docker compose.
Role documenation is updated.