common: get_command_descriptions use cout instead of dout

Using dout instead of cout creates noise and makes it more difficult for
people trying to debug a failure in test_ceph_argparse.py.

The output of get_command_descriptions is modified to only contain
parseable information and the code parsing its output
test_ceph_argparse.py is removed.

Signed-off-by: Loic Dachary <loic@dachary.org>