interfaces: ignore predefined network interfaces

interfaces: ignore predefined network interfaces

Some interfaces, like the one managed by libvirtd to provide a network
bridge to locally hosted virtual machines, can be completely ignored
when gathering ICE candidates. The motivation for adding this
possibility is that, ignoring them doesn't remove capabilities, and
improves the overall speed of the connection check method, by reducing
the number of pairs to be tested. This patch adds the possibility to
define such interfaces in the configuration script.

Differential Revision: https://phabricator.freedesktop.org/D948