Refactor robot tests to use p.a.robotframework and support Sauce Labs

Copy of pull https://github.com/plone/Products.CMFPlone/pull/63 to master as requested by @tisto

This doesn't include the following (other than robot related) commits from 4.3.x:
- https://github.com/plone/Products.CMFPlone/commit/65cfd1ffd1b8d1634823c65cd068d04297824f89
- https://github.com/plone/Products.CMFPlone/commit/e2465a27fa38151852f9a8d9c6acf352327a553e