Tezt_cloud: Layer1: Fix and improve config file mechanism

What

Following the addition of the config file mechanism to LAYER1 scenario, a few differences have been noted, which means deviating from the DAL scenario workflow. This MR aims to reduce that deviation.

Furthermore, with the introduction of compulsory "stake", "snapshot" and "network" arguments, simple commands like --help were not working anymore.

Why

To fix the --help command.

How

Manually testing the MR

You can try with this configuration file that uses all options available to the LAYER1 scenario:

{
  "dockerfile_alias": "ubuntu_plucky",
  "tezt_cloud": "gabriel_rionet",
  "scenario_specific": [
    "LAYER1",
    {
      "stake": [
        1,
        1
      ],
      "network": "rionet",
      "snapshot": {
        "local_file": "./rionet.rolling"
      },
      "stresstest": {
        "tps": 10,
        "seed": 1
      },
      "dal_node_producers": [
        2,
        3
      ],
      "maintenance_delay": 1,
      "signing_delay": [
        0,
        0.5
      ]
    }
  ],
  "artifacts_dir": "/tmp/tezt_cloud_experiment_1",
  "daily_logs_artifacts": true,
  "retrieve_ppx_profiling_traces": true,
  "teztale_artifacts": true,
  "website": true,
  "grafana": false,
  "monitoring": true,
  "localhost": true
}

with the command:

dune exec tezt/tests/cloud/main.exe -- LAYER1 -v --config-file layer1_config.json

Also, check that now the following command works:

dune exec tezt/tests/cloud/main.exe -- LAYER1 --help

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