Make network fixture management optional.

Whilst developing the integration tests for the vcloud-disk_launcher tool, I discovered that I would need to reference the 'network_1' and 'network_2' parameters, even though I did not need them for my tests.

This PR aims to fix that - so that the network fixtures will only be compared and created by vcloud-tools-tester if they are specified in the expected_user_params array passed into TestSetup.

This also means that this logic can be triggered on a per-integration-test basis too -- I have previously been bitten by this when writing the vCloud Core IndependentDisk class integration tests.
Wording this slightly more clearly, I think the constructor parameter is named correctly, as this is the data we have available and what we are basing our requirement for fixtures upon.

I do agree though that burying the logic of what fixtures we subsequently create on a stored copy of this array (in @expected_user_params) is misleading.

Maybe we could leave that for a future refactor though?