Run flake8 on all the CLI plugins

See https://trac.openmicroscopy.org.uk/ome/ticket/2591

This PR:
- fixes all the CLI plugin files to remove all the `flake8` warning
- add flake8 on `components/tools/OmeroPy/src/omero/plugins` to the Travis build
- review the usage of the multi-line strings in the `help` of the plugin parsers

The Travis build should be green and no change of functionality should be expected across the CLI plugins