[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.

from os.path import join
assert __file__.endswith(join('scripts', 'script2.py')), __file__
assert '__main__' == __name__, __name__
assert ['-v'] == __args__, __args__