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.
It there value in retaining this test? It might be confusing to someone new to the project so I'm not sure if it's worth keeping (though it would have been useful during development).