Refactor ways of creating anonymous test user

Some Swift tests require random user creation to test ACL features or
prepare special data by reseller admin's requests, but that occurs
code duplication. Especially, assigning role id code was too lengthy.

This commit aggregates common codes and remove unused clean-up statements.

Change-Id: I4b965fceeec7c9cd1857375053e148d85f5f5bca
Merge "Refactor ways of creating anonymous test user"