vnstat does not work when eth0 interface is absent

[babluboy]

As described in #31, running vnstat shows a database not created error if eth0 interface is absent. Create a check for vnstat and manually create the database if no default database is found.

The commands for creating the dir and database for vnstat manually are here: https://www.2daygeek.com/install-vnstat-to-monitor-network-interface-traffic-on-linux/