Bug 2533: Fixed unexpected 500 error on path policy REST API.

  * Return 400 (Bad Request) error if link cost is not configured in
    PathCost element.
  * Added integration tests for path policy REST APIs.

Change-Id: I7ef5e5b144b2eb8456ba51840ec2c067320b0f9e
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>