PPP config loses "on-demand" setting when configured via interfaces tab
I configured a PPP interface to run a verizon LTE modem plugged into a USB port. It is assigned as "WANVZ"
If I go to Interfaces -> assign -> PPP -> edit my VZ modem, in the advanced tab I can set dial-on-demand and set the timeout to 120 seconds. This seems to work ok.
If I go to Interfaces -> WANVZ, and I click save, the on-demand settings are erased.
I think this is a bug, as it silently removes an important setting.
Based on discussions with jimp in support ticket IAV-77363, I believe that the WANVZ interface page is causing blank values to be stored for the items in the PPP Advanced settings.
Fixes #3727 Do not unset ondemand for ppp type interfaces since it is controlled here only for pppoe/l2tp
Fixes #3727 Do not unset configuration values from ppp config if not needed.