ceph-volume: update argparse help output assertions for python compatibility

ceph-volume: update argparse help output assertions for python compatibility

This commit updates test assertions to be more flexible about
argparse help output format. It replaces checks for 'optional
arguments' and 'positional arguments' with checks for 'positional'
and help flags ('-h' or '--help'), which works across different
python versions where argparse output format may differ.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>