feat/refactor_args

[uggla]

• Refactor completed.
• This will be useful/needed for Riemann mTLS.
• Goal is to define args on the exporters to allow a maximum
  flexibility. (usage of various options like group options etc...)
• Avoid the code in main to choose between args with default values and
  no default values as this is not flexible at all.

 if let Some(default_value) = &opt.default_value {
                let arg = Arg::with_name(key)
                    .required(opt.required)
                    .takes_value(opt.takes_value)
                    .default_value(default_value)
                    .short(&opt.short)
                    .long(&opt.long)
                    .help(&opt.help);
                subcmd = subcmd.arg(arg);
            } else {
                let arg = Arg::with_name(key)
                    .required(opt.required)
                    .takes_value(opt.takes_value)
                    .short(&opt.short)
                    .long(&opt.long)
                    .help(&opt.help);
                subcmd = subcmd.arg(arg);
            }

• All help outputs have been checked and are identical as the ones before the refactoring.

[uggla]

Finally I did it completely, it makes the code simple and more flexible. However it will conflict with #98.
@bpetit, I don't know which MR should be merged first between #98 and #100, but maybe this one first will cause less rework.
If you merge this one, I will rebase #98 and fix -p / -r behavior as soon as we agree about it.

[bpetit]

This is great, thanks. I agree on merging this one too before merging #98
The CI server has a weird behavior, I have to fix that. In the meantime, I just ran the whole test procedure on a machine of mine. If it's successful, I'll merge this one.

[uggla]

👍 😸