Previously the way to define the log level is through an option to the
root `swh` cli interface.
It means that this syntax has to be used:
```
$ swh --log-level DEBUG <COMMAND> <ARGS>
```
while this one did not work:
```
$ swh
<COMMAND
> --log-level
<DEBUG
> <ARGS>
```
As the `swh` command is just a router to sub ones defined in other swh components,
I think the log level option should be available at the sub-command level
and only for those that make use of logging.
So prefer to define a decorator that should be used to declare the log level
option in any subcommand that needs it.
I also did the following:
- remove the use of private members from the logging module
- add missing help options when calling the `swh-db-init` command