Cinder and cinderclient assumes an attachment create request will
always contain instance_uuid. This is not true when glance calls
cinder for attachment in glance cinder configuration.
This patch (along with the cinder patch) make the instance_uuid
optional and allow glance to do attachments without passing
instance_uuid.
Change-Id: Ifbaca4aa87d890bc5130069638d42665b914b378