Fix unit tests for RHEL

Fixes bug 1158609
Fixes bug 1158612

Change-Id: I5ee6214a935cfa663c96b1ba6893f265890c4202
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
Merge "Use wrappers instead of direct calls to ip route."