Fix CLI args precedence in Client/Signer/Aggregator

[jpraynaud]

Issue

The arguments passed through environment variables in the CLIs are ignored when a default value for a clap argument is set.

Task

• Fix the precedence order of selection of arguments (or remove default values in clap)

[ghubertpalo]

In the Client, we shall not have to provide GENESIS_VERIFICATION_KEY unless restoring.