[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.
usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl