[readme] use openstackclient to delete users
[readme] use openstackclient to delete users

The python-keystoneclient CLI is deprecated in favor
of python-openstackclient, python-openstackclient should
be used to remove users.

Change-Id: Ib2291526263c6c8de5cc602a82e52391aa9e526e

stackforge/ospurge,openstack/ospurge,openstack/ospurge