OpenStack Magnum relies on specific versions of Fedora CoreOS, this
patch addresses this maintaining a map of images per release according
to the upstream documentation[0]
The images are expected to be stored in the object store pointed out by
TEST_SWIFT_IP[1] environment variable in a container named 'magnum'. A
bash script to upload images can be found at
./zaza/openstack/charm_tests/magnum/upload_fedora_coreos_images.sh
[0] https://docs.openstack.org/magnum/latest/user/index.html#supported-versions
[1] https://github.com/openstack-charmers/zosci-config/pull/262