Enable policy control over external_gateway_info sub-attributes

Part 2 of blueprint l3-ext-gw-modes

This patch extends the logic for building policy rule matches in order to
include sub-attributes as well. This logic will be leveraged by the
ext-gw-mode api extension.

Change-Id: I7f46a395597b71bb1c5110aa4e792a04a5010d4c
Merge "Enable policy control over external_gateway_info sub-attributes"