Deprecate deployment of MongoDB

Per http://eavesdrop.openstack.org/meetings/kolla/2020/kolla.2020-02-12-15.01.txt

Deprecates support for deploying MongoDB. In Victoria support for
deploying MongoDB will be removed from Kolla Ansible. Note CentOS 8
already lost support for MongoDB due to decisions made upstream.

This affects Panko as it will no longer be possible to get automatic
deployment of MongoDB database for it. However, the default, SQL,
backend is and will be supported via MariaDB.

MongoDB lost its position in OpenStack environment after
controversial relicensing under their custom SSPL (Server Side
Public License) which did not pass OSI (Open Source Initiative)
validation.

Change-Id: I09f77d275dfd2c8f9ae97a47c8ab1136a8de880f