[openstack|compute] images collection should not return nil for #all

Fog::Compute[:openstack].images.all returns nil without the patch.
The patch should bring the images collection behaviour in-line with
other collections I think.

The added test should expose the issue.
Merge pull request #1624 from rubiojr/openstack-compute-images-fixes

[openstack|compute] images collection should not return nil for #all