Flake8 omero.cli

See https://trello.com/c/vnojR45Q/43-flake8-omeropy. This PR modifies `components/tools/OmeroPy/src/omero/cli.py` to let it comply with the PEP-8 style.
This PR will need coordination with #2902 to unexclude `omero/cli.py` from `setup.cfg`. Nevertheless I am opening it so that we test it extensively in the merge build in the upcoming weeks.

To review this PR:
- run flake8 on the file (overriding `components/tools/OmeroPy/src/setup.cfg`) and check it is clean
- check the integration tests are still passing
- test the CLI commands (should I list them?)

/cc @ximenesuk