Calculate a suitable column width for positional arguments

Overwrite the method add_arguments in the class
OpenStackHelpFormatter to calculate a suitable columnt width
(max_help_position) for the positional arguments.

example output before this patch:

    alarm-threshold-create
                        Create a new alarm based on computed statistics.
    alarm-threshold-update
                        Update an existing alarm based on computed statistics.
    alarm-update        Update an existing alarm (Deprecated).

example output after this patch:

    alarm-threshold-create    Create a new alarm based on computed statistics.
    alarm-threshold-update    Update an existing alarm based on computed
                              statistics.
    alarm-update              Update an existing alarm (Deprecated).

Change-Id: I945e46bb9e0309b960e3322261d9d6ff9fa35b52
Closes-Bug: #1326471