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
Add swap measurement unit (MB) to CLI output

The measurement unit of the swap memory in flavor is MB.
If not clearly labeled, some may confuse it with other disk's
units which are GB.

Closes-Bug: #1323541
Change-Id: I29b4c8e14bcdb3505d5c0bae8bcf653c96b27413
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 "Add swap measurement unit (MB) to CLI output"
Merge "Overwrite HelpFormatter constructur to extend argument column"
Merge "Change help for --poll option in Nova commands"