Make Plinth listen on IPv6 local address too by default

Created by: SunilMohanAdapa

Plinth is does not listening on an IPv6 address. This is not currently a major issue because Apache is the only thing that contacts Plinth and it does so using and IPv4 address. However, it is good have Plinth listen on a IPv6 address also.

Freedombox-setup comes with a test suite that checks whether Plinth listens on IPv6 address. Currently, this test is failing.