Run flake8 on all the CLI plugins (rebased onto dev_4_4)



This is the same as gh-1685 but rebased onto dev_4_4.

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Fixes https://trac.openmicroscopy.org.uk/ome/ticket/2591 for the CLI plugins

To test this PR:
- install flake8 (`pip install flake8`) and check

 ```
flake8 components/tools/OmeroPy/src/omero/plugins
```
  does not display any warning. 
- check the Travis build is green
- test the CLI plugins do not lose functionalities. Note: several CLI tests PRs will likely be opened in the upcoming month. We can likely wait for these tests to appear and show green in the daily integration job before merging this PR.