change deprecated call in fog openstack access test in "Validate your OpenStack" article

I've got this warning message every time I refer to ```OpenStack``` object:
```
[WARNING] OpenStack[:compute] is not recommended, use Compute[:openstack] for portability
```
this is fixed by changing it to ```Compute[:openstack]```. I use ```fog (1.14.0)``` gem for the test.

This pull request is covered by contribution agreement between Altoros and Pivotal.