Enable cross-testing of oslo.test with dib and t-i-e

I'm planning to start using oslo.test in diskimage-builder and
tripleo-image-elements soon, so I would like to have cross-testing
enabled for those projects.

Change-Id: I3e213ea92c84547e42f6659b7a22f6f522927182
Merge "Enable cross-testing of oslo.test with dib and t-i-e"