(FACT-593) Remove dependency on route binary from dhcp_servers fact

Previously, the dhcp_servers fact assumed that the route binary was present, which it then tried to use to determine the primary interface. This commit updates that fact to instead use /proc/net/route to retrieve the same information.
I'd rather see us return nil instead of this to indicate there's no default gateway_device, personally (and take account of that in where we call `gateway_device`).