[services] don't hardcode 'users' group for register_user Let the caller choose which group the user should be in. Related to #3020639 Caveat: this is a possible security hole if untrusted values are allowed.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances