[sepia] teuthology is now python3

hi folks, we just migrated ceph:teuthology and all tests under qa/ in ceph:ceph to python3. and from now on, the teuthology-worker runs in a python3 environment by default unless specified otherwise using "--teuthology-branch py2". which means: - we need to write tests in python3 in master now - teuthology should be python3 compatible. - teuthology bug fixes should be backported to "py2" branch. if you run into any issues related to python3 due to the above changes, please let me know. and i will try to fix it ASAP. currently, the tests under qa/ directories in ceph:ceph master branch are python2 and python3 compatible. but since we've moved to python3, there is no need to be python2 compatible anymore. since the sepia lab is still using ubuntu xenial, we cannot use features offered by python3.6 at this moment yet. but we do plan to upgrade the OS to bionic soon. before that happens, the tests need to be compatible with Python3.5. the next step is to - drop python2 support in ceph:ceph master branch, and - drop python2 support in ceph:teuthology master. - backport python3 compatible changes to octopus and nautilus to ease the pain of backport -- Regards Kefu Chai