Merge branch 'master' into erp5-cluster
s.r.apache-zope-backend: Add support for multiple listening IPs.

Minimal change, just made to listen identically on IPv4 and IPv6 addresses.
Settings are identical across all IPs.
erp5: Make backend apache also listen on IPv6.

Also, fix frontend request.
erp5: Drop frontend-related documentation.

Frontend support is desired, but current option set is too weak to cover
even simple cases.
erp5: Stop using slapos.cookbook:haproxy .

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).