Extract the user and group membership creation behaviour from
DogtagInstance.setup_admin to its own method, 'create_user'. The
ACME setup routine will use it to create ACME RA accounts.
The @staticmethod decorator documents that 'create_user' does not
use 'self' or 'cls'. I preferred not to lift to a top-level def
because it is very much a "DogtagInstance" behaviour.
Part of: https://pagure.io/freeipa/issue/4751
Reviewed-By: Rob Crittenden <rcritten@redhat.com>