disable networking in Copr by default

[Pavel Raiskup]

Just to be more pedantic and probably bit more secure, what about making
networking in copr opt-in (at least for new projects)?

If build of my package used internet arbitrarily, I would be warned very
early..

Also, in Koji we also do not have networking, so we can faster teach
(future) Fedora packagers.

Thanks for considering,
Pavel