Fix the MongoDB description

Now, the MongoDB is just used as Panko backend[0]

[0]: https://github.com/openstack/kolla-ansible/blob/master/ansible/group_vars/all.yml#L630

Change-Id: I8cfd4d66bf27ce754e3fef610d613e7e42740952

README.rst

@@ -105,8 +105,7 @@ Kolla-Ansible deploys containers for the following infrastructure components:
 - `MariaDB and Galera Cluster <https://mariadb.com/kb/en/mariadb/galera-cluster/>`__
   for highly available MySQL databases.
 - `Memcached <https://memcached.org/>`__ a distributed memory object caching system.
-- `MongoDB <https://www.mongodb.org/>`__ as a database backend for Ceilometer
-  and Gnocchi.
+- `MongoDB <https://www.mongodb.org/>`__ as a database back end for Panko.
 - `Open vSwitch <http://openvswitch.org/>`__ and Linuxbridge backends for Neutron.
 - `RabbitMQ <https://www.rabbitmq.com/>`__ as a messaging backend for
   communication between services.