The description of the volumeAttachment request parameter was actually
the description of the response parameter, which included more fields
than we allow on the POST and PUT requests for os-volume_attachments.
This fixes the descriptions for both POST and PUT and also includes a
tiny fix for the wording on the existing volumeAttachment parameter.
Change-Id: I4ccd4ac12e24b232925875fdb5fb568c2bfaf417
Closes-Bug: #1675536