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.
This shouldn't need changing as there's no changing in behaviour in `UserParameters`.

I don't really object to the variable renaming here, but the changes in `user_parameters.rb` and its spec tests don't match the commit message in d62912a2. I think `UserParameters` should be left as-is.