network startup: Only work with configured zones

network startup: Only work with configured zones

Otherwise the called script fails with an failure message.
This does not hurt but is ugly.
I also changed my mind about the wording here. Because we only have four
possible zones we could talk about valid zones. But I think it is easier
and more future proof to talk about what is configured on the system.

Should make further changes easier.

Signed-off-by: Jonatan Schlag <jonatan.schlag@ipfire.org>