Refactor CLI prefs unit tests to use pytests capsys fixture

- Drop MockCLI to use CLI instance itself
- Use capsys to systematically check the sys.stdout and the sys.stderr
Fix flake8 for the clitest unit tests
Add test/unit/clitest to the directories to flake8 in .travis.yml
Use absolutepath for setuptools in setup.py
Add cleanup() methods to test_hdfstorage.py
Merge pull request #1821 from sbesson/cliunittest

Unit tests (CLI tests flake8 and Travis fix)