Resolve public IPv4 interface

Summary:
Currently, the node.resolve_network function indicates the IPv4 address,
but not the interface it uses.

That information is needed for firewall configuration.

Test Plan: salt '*' node.resolve_network

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3548