Be able to prevent startup via config

Currently if the mongodb package is installed then the server always runs. I would like to have the package installed (ie the software is ready) but not running. By convention the way this is done is by a file such as /etc/default/mongodb that would contain a line like:

ENABLE_MONGODB=no

The startup will then be prevented. To see an example of this being done on Ubuntu, install the memcached package and look at /etc/default/memcached