Change help message for volume-update

`nova volume-update <server> <volume> <volume>` to
`nova volume-update <server> <attachment> <volume>`

Change-Id: Ie820843c3e8d23659075c1d6b4fdd47f639b3b58
Closes-bug: 1326613
Remove _print_volume from volume-update

Since the response of volume-update do not contain a body, just a status
code 202, so when calling _print_volume(volume), there will be an error:
`ERROR (AttributeError): 'NoneType' object has no attribute '_info'`.

Change-Id: I950ae7dca5e847beae7295d72d009f2d07d07c51
Closes-bug: 1321205
Merge "Remove _print_volume from volume-update"
Merge "Change help message for volume-update"