expect ip_addresses as an array instead of a single entry, as we support multiple servers

should fix #2071317, but untested
move subdir implementation to the Apache service

the reasoning is that we need to operate on multiple servers, which
can't be done outside of services.

furthermore, we had to move to a Config class for sync to work, and we
hack up that class badly so that it is forced to generate multiple
configuration files instead of just one, which is impure and weird,
but seems to work in my first tests.

we still need to reimplement sites.php and disabled sites support, but
multi-server should work better now

see #2020075 and #2020079
fix disabled site support
restore crappy vhost and sites.php support
no need to regenerate on enable now that disable/enable is handled by the service
move subdir vhost generation to the service
fix path to vhost file
fix multiple vhost support for subdir
remove duplicate uri
subdir: reuse the same variables as the usual vhost template
subdir: fix site_path
Merge branch '6.x-2.x' into dev-subdir-multiserver
load subdir templates only if enabled
fix typo
fix message
Issue #2098289 by helmo: Fixed Backup task does not detect failed database dump.
remove cruft
subdir: add missing check to operate only on sites
subdir: reshuffle code now that we manage only symlinks
properly detect IP from the frontend, fix warning