Add subcommand parsing capability
Mistral client is currently unable to parse the sub-commands.
if the user asks for help on a particular use case, say, workflow-create,
$ mistral workflow-create --help
All the mistral usecases are being displayed
Only the help message for workflow-create usecase must be displayed.
"mistral COMMAND --help" must be the same as "mistral help COMMAND" as this is the behavior in all the other projects.