Fix some usage messages of 'nova volume-*'

'nova volume-show', 'nova volume-delete', 'nova volume-snapshot-show'
and 'nova volume-snapshot-delete' can be specified by ID or name as
the argument.
But that is not described on the usage messages.
This patch fixes the usage messages.

Fixes bug 1093172

Change-Id: I3dcdd2aca923e9304efd9ea7b467d5b81cd642a7