Really do not lose the default route when editing an interface

[bnc#883836#c15](https://bugzilla.novell.com/show_bug.cgi?id=883836#c15)
Ah, I mean http://ruby-doc.org/core-2.1.2/Hash.html#method-i-invert
In general idea is to have
`TRISTATE_MAP = { nil => nil, false => "no", true => "yes" }`
and in different direction use `TRISTATE_MAP.invert.fetch(key, nil)`