Tezt: drop Cli.init, TPS: refactor the app

As requested by @romain.nl, here is the change that eliminates the problematic Cli.init function from the Tezt framework. Instead of using commands with cmdliner we now use Tezt tags to select the right "command". We pass options by using the newly conceived --test-arg or -a command line option.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR