Update galera_wsrep_sst_auth_user to use galera_root_user
Update galera_wsrep_sst_auth_user to use galera_root_user

With the changes to the root user and creation of the admin user in:
https://review.opendev.org/c/openstack/openstack-ansible/+/775684/
galera_wsrep_sst_auth_user needs to use the overridden galera_root_user
as the password is no longer set on the root user.
Otherwise galera cannot sync and cluster properly as access is denied.

Change-Id: I8f03ee7a7a144fa901caf7b6c1ed041e09f2ffc0

openstack/openstack-ansible-galera_server,openstack/openstack-ansible-galera_server