luci-app-openwisp: improved schema for usability

Without these changes, saving the form of luci-app-openwisp without
doing any change generates a configuration which will not work in most cases.

In this patch I took care of providing the right settings to show the default values,
avoid saving default values and show the interface names properly.
With these changes it will be possible for a user to edit just 1 field without
introducing breaking changes.

Signed-off-by: Federico Capoano <f.capoano@openwisp.io>