`require "opennebula"` fails when downstream tries to require `xmlrpc`
which is not included within the main `fog` dependencies.
The problem is that the `LoadError` was being trapped by
`Fog::SevicesMixin#new` so it was reporting that the OpenNebula provider
did not offer the service.
This then handles the `Fog::Errors::LoadError` in all the shindo tests
that halted the run. These tests will be pending unless `opennebula` is
installed.
