Add tests for ovirt-img sub commands

[aesteve-rh]

We meed more tests to increase coverage and avoid breaking on updates for the recent ovirt-img command.

The complexity is that it requires an engine.

One option is add integration tests that contributors need to run manually with engine, or that will run if you enable the target:

tox -e integration

The test can create an image with virt-builder, upload it to configured storage domain, and download it. We need some kind of local configuration that:

1. enables the integration tests
2. provides the storage domain name