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.
`expected_user_params` should probably be named `expected_fixture_params`.

`user_params` are user-defined, whereas `@fixture_params`, exposed by the `attr_reader`, are returned by vcloud-tools-tester when the fixtures are created.

We're really specifying what should be returned in `@fixture_params` so I think we should be careful to distinguish between the two. Does that make sense?