'server_id' is not Attachment attribute, should be
set by 'instance' attribute.
v3/attachments respond body:
{"attachments":
[{"status": "attached",
"instance": INSTANCE_UUID,
"id": ATTACHMENT_UUID,
"volume_id": VOLUME_UUID,
},
...
]
}
Change-Id: Ica5d278cb7455befe1db4be0ab65114fd606ea0a