Add setup.cfg file at the root of OmeroPy test/ and simplify Travis (rebased onto develop)



This is the same as gh-2287 but rebased onto develop.

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As @ximenesuk is starting to validate the integration tests to comply with `flake8`, this PR proposes to add a `setup.cfg` at the root of the `components/tools/OmeroPy/test` folder. 
This way:
- `flake8` can simply be run as `flake8 -v components/tools/OmeroPy/test` 
- files validation can be  checked by updating the exclude list and re-running the command above
- no further change needs to be added to the validation of the `test` folder in the Travis script