Add python3 support to our various scripts.
(Note that if the system default python is python2, the call to getmanifests.py will use python2, so test-common.py must continue to support python2. But when setup-user-info.py goes to add ssh keys via the novaclient library, for release >= Stein, that is python3. Thus test-common.py must also be python3- compat.)
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