Provide default value for network

https://gitlab.developers.cam.ac.uk/uis/devops/raven/deploy/commit/3baea44beddfb8779b82c0deb0fde69d240c27aa added a workaround to fix a problem where this module would repeatedly re-create the k8s server. In that commit it was suggested that a default value for the network variable could be used to stop the module re-creating servers.