Disable Mongo HTTP interface

MongoDB by default provides an HTTP interface running by default on TCP
port 28017 which provides the “home” status page. This provides a vast
amount of information and the interface is not recommended for
production environments.
Merge pull request #623 from alphagov/feature/disable-mongo-http-interface

Disable Mongo HTTP interface
Pin our version of mongodb

We recently reprovisioned our mongo machines, and by a happy accident
upgraded to a more recent version.

This change explicitly controls which version we’re using, so that it if
we add more nodes, they will all be running the  same version until we
choose to upgrade.