Use wrappers instead of direct calls to ip route.

Use wrappers instead of direct calls to ip route.

 - extract the logic of ip route wrapper into a separate
   class to drop dependency on a specific network device
 - add route wrapper to IPWrapper class
 - use IPWrapper instead of direct calls to ip route in l3 agent
 - update tests

Fixes bug 1133133.

Change-Id: Ic7174b0676d1a565909bb5f6f950376cf8fae8d2