[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/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/