Update the database connection for keystone

1. the pymysql is needed for the current release[0]
2. it should use keystone_database_address rather than
keystone_service_ip, because they may different.

[0]: https://github.com/openstack/kolla-ansible/blob/master/ansible/roles/keystone/templates/keystone.conf.j2#L22

Change-Id: I7eb90544bd6be167df157ca352ca01e21ce781bd