Improve the description of -l option

[gigony]

If we add logging option -l DEBUG to the monai-deploy package command, it would affect only for packager module (it would show the messages during executing docker build command), not app itself.

However, it is possible that the user may think that specifying -l DEBUG would make App's default logging level to DEBUG when it's not.

It is good to improve the description of -l option.

Since -l option is from main CLI parser(monai/deploy/cli/main.py) that packager parser(monai/deploy/packager/package_command.py) cannot control, we may need to clarify it in main CLI parser's description of -l option or in the documentation.

[MMelQin]

Additionally, I recall why I said the --tag was stated to be optional: it is the package --help which says so, as one of the optional arguments with a nice default.

 optional arguments:
  -h, --help            show this help message and exit
  -l {DEBUG,INFO,WARN,ERROR,CRITICAL}, --log-level {DEBUG,INFO,WARN,ERROR,CRITICAL}
                        Set the logging level (default: INFO) (default: None)
  --tag TAG, -t TAG     MONAI application package tag (default: None)
  --base BASE           Base Application Image (default: None)
...

Apparent the help text is wrong, as the --tag in the Packager's speaking is the name and tag of the docker image.