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.
I had changed expected_user_params to be expected_params everywhere, since the same *data* is being passed around these classes from the TestSetup constructor. TBH, I thought this is what you wanted.

If it's going to block this PR, then I can rebase out those changes, but it's going to mean we lose the conversation on this PR as I'll need to open another.

If you'd like a new commit that puts things back then I'm happy to do that, it just seems a bit unnecessary.