Call vcloud-logout after integration tests

To guard us against the possibility that our vCloud session tokens might be
exposed and copy/pasted from CI build output. I'm using a `trap` to ensure
that this gets called regardless of whether the spec or integrations tests
fail.
Release 3.6.0

So that the following are available to users:

- vcloud-logout utility: gds-operations/vcloud-core@ab465cc5acec906120d6713e2a2ae4e3d1138c19
- Fog bugfix for usernames: gds-operations/vcloud-core@f52d618638b27a5bf2998f8cba133fcb5e81f33e