Bug 2536: Fixed bug that path policy APIs never remove flow entries.

When a path policy is created, modified, or removed, the VTN Manager
should remove flow entries affected by the target path policy.

Change-Id: I98f79869c37ede42fcaddfb2c45bd02873bb89c7
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>
(cherry picked from commit e98ee28a7ac7b50a1677dea4a649008b5dbc30d9)