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:

    ec2-credentials-create
                        Create EC2-compatible credentials for user per tenant.
    ec2-credentials-delete
                        Delete EC2-compatible credentials.
    ec2-credentials-get
                        Display EC2-compatible credentials.
    ec2-credentials-list
                        List EC2-compatible credentials for a user.

example output after this patch:

    ec2-credentials-create  Create EC2-compatible credentials for user per
                            tenant.
    ec2-credentials-delete  Delete EC2-compatible credentials.
    ec2-credentials-get     Display EC2-compatible credentials.
    ec2-credentials-list    List EC2-compatible credentials for a user.

Change-Id: Ibd5d3502022d2afcfd4f57fef9a0bbf23d34b767
Closes-Bug: #1326471
Merge "Calculate a suitable column width for positional arguments"