Refactoring of contrib.test_networks tests

Some simple refactoring of contrib.test_networks tests in order
to make it easier to share unittests between v2 and the v2.1 API
in a following changeset

Partially implements blueprint v2-on-v3-api

Change-Id: I87e475acdaabacdcceaaf73b3ad831292644d3c7
Merge "Refactoring of contrib.test_networks tests"