controller/tunnel: introduce new class for tunneling
controller/tunnel: introduce new class for tunneling

This should be renamed (already used for non tunnel stuff, e.g. VLAN).

Based on the following patch:

From: Isaku Yamahata <4b3abd66323411eeceff090ab3754dd766fc4b25@valinux.co.jp>
Subject: controller/tunnel: introduce new class that tracks infos related to tunneling

- helper functions and event generator for gre tunnel
- plug events for gre tunnel app

Signed-off-by: Isaku Yamahata <4b3abd66323411eeceff090ab3754dd766fc4b25@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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