Add path map feature.

Path map defines the packet routing policy in the physical network.
A path map keeps a pair of flow condition and path policy.

  * Flow condition specifies the condition to select packets.
  * Path policy implements cost-based packet routing.

Change-Id: I32692a90e636529e9c14c4223e87b0666efd079b
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>