Adding tests for images

Finally a complete version of the image testing.

Had to add some plumbing to avoid certain issues. Some tests were added to the skip list as I could not get these to work.

• For upstream tests I added a mirror: https://gitlab.cern.ch/linuxsupport/testing/centos_functional_tests
• For CERN specific tests: https://gitlab.cern.ch/linuxsupport/testing/cern_centos_functional_tests

We will keep adding CERN specific tests.

https://gitlab.cern.ch/linuxsupport/openstack-testing-cleanup takes care of removing long lasting testing VMs from failed tests. I will soon add something to remove testing images as well.