(cherry picked from commit af181db0)

It is superseded by per-zope value, published by Zope partitions.

Fixes backend maxconn value: it's now equal to zope's number of threads.
Instead, generate configuration file from template and wrapper from
stand-alone recipe.
One local-only feature is lost: haproxy-ctl command is not present, but its
usage should be rare enough to not be a problem now. Socket is present
anyway, so any tool able to access a unix socket would do (ex: socat).

Frontend support is desired, but current option set is too weak to cover
even simple cases.

Also, fix frontend request.

Minimal change, just made to listen identically on IPv4 and IPv6 addresses.
Settings are identical across all IPs.

It is useful to temporary enable such settings while preparing an instance.

This follows up commit 04ff4930

It prevents data loss while upgrading partition.

Because new s.r.templare requires new Jinja2.

Before, a random password was generated for each group of zopes.