Create a configuration file for tezt-cloud

Why

To avoid 5 lines long commands.

How

By creating a config file option in lib_cloud which is decompacted, at the cloud level, except the scenario_specific field, which is passed as raw json to the scenario which decode it.

Manually testing the MR

One can now launch:

make && dune exec tezt/tests/cloud/main.exe -- DAL -v --website --grafana --log-file /tmp/log_$(date "+%Hh%M") --producers 1 --localhost --tezt-cloud guillaumeg --config-file tezt/tests/cloud/config.json 

where tezt/tests/cloud/config.json contains:

{
    "dockerfile_alias": "dal",
    "scenario_specific": [
        "DAL",
        {"stake": [2,2]}
    ]
}

and see that the experiment is the expected one. (Of course the objective would be to put everything in the config file, but I ran only this experiment currently.