[vcloud_director|tests] Fixes and cleanup

This fixes a few obvious problems to increase (non-mocking) test coverage and removes some assumptions about the environment in which the tests are run (e.g. API host name) so it's now possible to run tests for real and re-record VCR cassettes.

I've left it as multiple commits because c1fc2c07e704a74e423f5ef009f3526e736fa198 might be contentious. The cassettes are no longer useful due to other changes, and as I've mentioned before, I'm not in a position to publish updated cassettes. So it makes sense to me, but it might not to others.