Make error more accurate if org missing

This caught me out when an organisation I was testing against was not
specified in my vCloud Tools Tester configuration file.

It took me a moment to figure out what the error actually meant, so I've
amended it to be more accurate.
Merge pull request #26 from gds-operations/correct-error-message

Make error more accurate when organisation is missing from configuration file
Refactor fixture details, for later rework

This quick refactor should make subsequent commits that
make specification of networks optional much easier to follow.

There is no functional change (yey refactor!), and unit tests
still pass.
Make network fixtures optional

In many integration tests, we do not need to have the network
fixtures present. This commit makes the requirement for having
a network fixture for (eg) network_1 or network_2 dependent
on the developer specifying "network_1" or "network_2" in the
expected_user_params array passed to TestSetup.

NB: This commit keeps the tests as-is for the 'both networks present'
    case, for simplicity. Futher tests and fixes for the 'no networks'
    case will be added in a subsequent commit.
Remove unneccessary instance variable

The @expected_fixtures_config was quite confusing. By removing it
we ensure that we're always referring to the correct data - we're not
prone to race conditions.
Handle the case where no fixtures are needed

If there are no 'network_1' or 'network_2' parameters
specified in our expected_user_params input, then this
flags that we do not need to worry about the presence of
these fixtures for this test.

This creates the valid situation where there are no @fixtures and
no fixture_params, which previously was considered an error.
Remove redundant instance variable
Remove transitional test

This test was put in place to confirm a change in behaviour, which
now is complete. Remove the test to tidy up the unit tests.
expected_user_params -> expected_params

After a discussion with @mattbostock, we agreed that expected_user_params
was the incorrect name for this variable - it's handling both user and
fixture params.

Rename to be less specific.
Update CHANGELOG
Rename expected_user_parameters in tests

As per ff8d741b5a3076932d7a1a9901d8acfdcec79c6e, rename the
expected_user_parameters variable to be expected_parameters
since this makes more sense now that it also relates to fixtures.
Merge pull request #27 from mikepea/optional_fixture_params

Make network fixture management optional.