Pytest markers registration (rebased onto develop)


This is the same as gh-2786 but rebased onto develop.

----

This PR:
- registers the pytest `--markers` option in `test_setup.py`. All registered markers can be listed via ``./setup.py test --markers`
- review and registers all defined pytest markers in the OMERO.py tests
- cleans some of the legacy markers used for testing (`cliobj`, `fs_suite`)

Next steps:
[ ]  define a marker for local integration tests: `in_place_import` ?
[ ] potentially enable `--strict` on the `py.test` to fail on unregistered markers

/cc @joshmoore @ximenesuk