Use gum-utils instead of shadow-utils for making security_test_user

Use gum-utils instead of shadow-utils for making security_test_user

Using plain useradd or userdel in %post scripts causes wrong label on
/etc/passwd and /etc/group files. The reason for that is that the user
manipulation tools from shadow-utils bakcup the old file in /etc/ and
create a new edited one instead of modifying it in-place.
This results in files in /etc created with Smack label of the process
that added or removed the user.

Change-Id: Id523dcd25d1434dbe2847db915d5b4f630b3861a