Move openstack-projects part from python2 to python3.
Add cloud archive.
Update qemu and libvirt versions to those from cloud archive (they
work with python3, while the distro packages versions don't).
Switch from rocky to stein.
Fetch libvirt and qemu sources via "apt source". Gets rid of sub
version hard coding in snapcraft.
Update hard coded references in tests to rocky from stein.
Refactor snap to work with core18.
Giving the snapcraft.yaml a base property helps tremendously with the
efficiency of the build process, and I believe that it puts us in a
better position to reliably support non Ubuntu distros going forward.
This also bases us on long supported bionic libraries, and gives us a
nice place to work from as we add Python 3 and Stein support, as well
as general polish and fixes.