ipaserver.install: Consolidate system user creation
Sytem users and their groups are always created together.
Also, users & groups should never be removed once they exist
on the system (see comit a5a55ce).
Use a single function for generic user creation, and specific
funtions in dsinstance and cainstance.
Remove code left over from when we used to delete the DS user.
Preparation for: https://fedorahosted.org/freeipa/ticket/3866
Reviewed-By: Tomas Babej <tbabej@redhat.com>