Refactor test_get_port_vnic_info 2 and 3

This patch addresses the comments left on review 121939 for
nova/tests/network/test_neutronv2.py, it removes duplicated code from
the added unit tests and corrects an identation issue.

Change-Id: I02c004297d4ee78e5695c3ca265052020f2fcfa3
Related-Bug: 1370077
Merge "Refactor test_get_port_vnic_info 2 and 3"