Route Tables: Update describe_route_tables to support additional route types (VPC peering connection, NIC).

This resolves #2597 by adding "interface_id" and "vpc_peering_connection_id" properties to Route, and parsing the corresponding XML data for said properties.

(Note: Both property names were chosen to match the corresponding kwargs in create_route/delete_route, thus "interface_id" rather than "network_interface_id".)