9953: Add arvados-docker-cleaner.service to MANIFEST.in.

9953: Add arvados-docker-cleaner.service to MANIFEST.in.

Fixes error during `setup.py install`:

Command "/tmp/tmp.lpB0zPHYWs/VENV3DIR/bin/python3 -u -c "import setuptools, tokenize;__file__=/tmp/pip-6gqvw70n-build/setup.py;exec(compile(getattr(tokenize, open, open)(__file__).read().replace(rn, n), __file__, exec))" install --record /tmp/pip-ljhdcczu-record/install-record.txt --single-version-externally-managed --compile --install-headers /tmp/tmp.lpB0zPHYWs/VENV3DIR/include/site/python3.4/arvados-docker-cleaner" failed with error code 1 in /tmp/pip-6gqvw70n-build/