machine: machine list Make name arguments optional and show all machines when it was not provided.

[karajan1001]

@karajan1001 Another minor UI tweak is that we need to make clear that the positional arguments are optional and all machines will be shown by default.

Originally posted by @dberenbaum in #6602 (comment)

[karajan1001]

@dberenbaum, Look into the code and the test on my local machine, I found that it had already been supported. And we had already had a test of it.

dvc/tests/func/machine/test_machine_config.py

Lines 95 to 102 in e2d705f

	assert main(["machine", "list"]) == 0
	cap = capsys.readouterr()
	assert "cloud=azure" in cap.out
	assert main(["machine", "list", "foo"]) == 0
	cap = capsys.readouterr()
	assert "cloud=azure" not in cap.out
	assert "cloud=aws" in cap.out

[dberenbaum]

@karajan1001 Thanks, but I meant here that we need to document that behavior in the help message.

Compare to dvc stage list --help output:

$ dvc stage list --help
usage: dvc stage list [-h] [-q | -v] [--all] [--fail] [-R] [--names-only]
                      [targets ...]

List stages.
Documentation: <https://man.dvc.org/stage/list>

positional arguments:
  targets          Show stages from a dvc.yaml/.dvc file or a directory. 'dvc.yaml' by
                   default