Merge branch 'feature/caddy-frontend-auth-to-backend'

Frontend can now do SSL client authentication to the backend. This is turned
off by default, but easy to enable by proper switch.

In order to achieve this result haproxy has been introduced as middleman just
before the backend. Also it came with rsyslogd, as haproxy can't log by
itself.

Documentation has been updated, also changelog has been introduced.

A lot of additional tests has been added due to adding additional component
(haproxy) which required a bit different ways to approach to the request input
data.

See merge request nexedi/slapos!771