gluon-core: fix handling of 'disabled' site.conf attributes for mesh interfaces

Because is_disabled() was always returning true or false, the
first_non_nil() would never actually check the default setting from
site.conf. This was broken since v2017.1.

Fixes: 6cf03bab ("treewide: replace normal uses of luci.model.uci with simple-uci to reduce LuCI dependencies")