Sleep in logout test because appears to be async

This integration test very occasionally fails with:

    1) Vcloud::Core::Fog#logout with a valid token should invalidate a previously working session
       Failure/Error: expect{ fsi.session }.to raise_error(
         expected Fog::Compute::VcloudDirector::Forbidden with "Access is forbidden" but nothing was raised
       # ./spec/integration/core/fog_spec.rb:29:in `block (4 levels) in <top (required)>'

It passes again on a subsequent run and is very difficult to reproduce. The
only reason for this that I can think of is that the `/logout` endpoint is
ever so slightly asynchronous and doesn't immediately revoke the session
after return a `204` response. It doesn't provide a task that we can monitor
the progress of so this hack seems to be the only way that we can guard
against the intermittent (and confusing) failure.
Merge pull request #148 from gds-operations/logout_async_test

Sleep in logout test because appears to be async