[network-topology] Add cron to run `update_topology` command

Created by: nemesisdesign

#19 introduced the openwisp-network-topology module developed by @rohithasrk but we forgot to add the cronjob to run ./manage.py update_topology which is needed to automatically update the topologies using the FETCH strategy every x minutes.

When openwisp2_network_topology is set to true we should add a cron that runs the update_topology command, eg:

3 * * * * /root/.virtualenvs/ninux_graph/bin/python /var/www/ninux_graph/manage.py update_topology

In order to control the cron frequency, we could copy the syntax used by the role finn.letsencrypt (which we suggest in our README to handle the SSL cert).