interfaces: assignments should take OpenVPN into account
interfaces: assignments should take OpenVPN into account

So 5f6b380a33 made get_interface_list() work as expected at the
price of losing assignment options on the referenced page due to
historic oversight.  Add a prefix so we can construct the device
name from a partial key value.

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core