Ussuri release is the one in which we drop Python 2 support, as its EOL
is pretty close now. This commit does so in kuryr-kubernetes by
removing Python 2 unit test jobs, switching all tempest jobs to Python
3, removing specific jobs for Python 3 and updating Dockerfiles to
centos:8 that includes Python 3 from the box.
Also CentOS 7 job is removed from check queue as it seems it doesn't
play well with Python 3. A CentOS 8 job will get created soon.
Change-Id: Id9983d2fd83cef89e3198b2760816cf4a851008b
28b27c5de2