Change help for --poll option in Nova commands

This patch clarifies the help about --poll option in these nova commands:
image-create, boot, migrate, resize, rebuild and reboot

Change-Id: Ia877fe5d07f2e9e60104f117c4a6032b6c419282
Closes-bug: #1312747
Overwrite HelpFormatter constructur to extend argument column

Overwrite the HelpFormatter constructor in the class
OpenStackHelpFormatter to modify the default value of the
max_help_position to extend the width of the argument column
in the help output.

example output before this patch:

    volume-snapshot-create
                        Add a new snapshot.
    volume-snapshot-delete
                        Remove a snapshot.
    volume-snapshot-list
                        List all the snapshots.
    volume-snapshot-show
                        Show details about a snapshot.

example output after this patch:

    volume-snapshot-create      Add a new snapshot.
    volume-snapshot-delete      Remove a snapshot.
    volume-snapshot-list        List all the snapshots.
    volume-snapshot-show        Show details about a snapshot.

Change-Id: I517098748947e7bded03160a15e778bf79475d0d
Closes-Bug: #1326471
Merge "Overwrite HelpFormatter constructur to extend argument column"
Merge "Change help for --poll option in Nova commands"