Clean up image (stream) in external deploy test

[michaelsauter]

When testing the external deploy, the image (stream) is not cleaned up after the test. This should be fixed. However it is not trivial to do so:

• skopeo delete does not work (see Cannot delete tag on hub.docker.com {"errors":[{"code":"UNAUTHORIZED","message":"authentication required" ...]} containers/skopeo#531)
• Using oc assumes the test runs against OpenShift

This is a follow-up from opendevstack/ods-pipeline#219.

[michaelsauter]

Transferring as a consequence of opendevstack/ods-pipeline#722.