mwan3_get_weight_member() collects wrong/multiple interface names

Created new issue as this wasn't precisely what my problem was in issue #8.

Let me know if this resolves your issue, Adze and if not I'll keep brainstorming.

Cliff's Notes:
1. mwan3_ifupdown() writes $INTERFACE, $DEVICE and the default route to temp file
2. mwan3_policy_routes() gets device name and default gateway of existing route and checks the temp file for a line containing both and sends that to mwan3_get_weight_member()
3. mwan3_rules_iptables() also checks the temp file for a line with the device name and gateway and feeds that interface name to mwan3_get_weight_member()