Using fixed ip to create vm

1.Adding code to create port with fixed ip in vn_fixture
2.Modifying the test case test_ping_within_vn_two_vms_two_different_subnets
  to use fixed ip to bootup vm

Change-Id: I6d07adc7c7d9d52f27b7539b3971bc3313a16c76
Merge "Using fixed ip to create vm"