[#65363454] Use vcloud tools tester

For discussion.

This PR the way you pass parameters to the integration tests. Instead of using environment variables, you use a config file and [vCloud Tools Tester](http://rubygems.org/gems/vcloud-tools-tester).

See the [README](/spec/integration/README.md) for more details.

It means that all tests can be run using a config file that we can share. It also means the only environment variable we need to set is the FOG_CREDENTIAL, so that removes the issue of parameters from one test interacting with those from another, and it also makes explicit the test parameters we are using.

I have tested this against two Skyscape environments and the config for them is [on GHE](https://github.gds/gds/vcloud-tools-testing-config/blob/master/vcloud_tools_testing_config.yaml).

The main reason not to merge this is that CI will fail, as that is still expecting to use environment variables. I welcome suggestions on how to handle that. If nothing else, we can construct a config file using the env vars that are there, but surely there is a way to pull in a file from a different repo for config.

I also realise that I haven't updated the README to remove the old instructions.

Potential improvements to vCloud Tools Tester include:

- adding a method that confirms all the parameters required for the test are set so there's one central place to test that (some tests do, some don't, some duplicate, some use a helper)
- a helper for retrieving the parameters? Or retrieving some of them for you and not requiring you to specify them? We have to be careful with this to make sure we are testing what we say we are testing, but there are certainly ways we could make it easier.

Comments very welcome.