Refactor test_scripts.py to use new CLITest infrastructure

- Fix flake8 warnings
- Remove unnecessary UploadControl registration
Add tests for script subcommand help option
Add integration test for script demo CLI command
Use pytest parametrization for the script subcommand help tests
Add flake8 validation for the CLI integration tests to .travis.yml
Remove unused omeroDir variable definition
Remove commented lines
Remove commented print statements
Remove unused packages
Merge pull request #1715 from sbesson/4335_clitest_script

Update CLI script integration tests