Fix element miss in client request body

Currently the client sends a request with wrong element name
'os-multiple-create:reservation_id' in the body.
The correct element should be 'os-multiple-create:return_reservation_id'.
The information can be got in this website.
https://wiki.openstack.org/wiki/NovaAPIv2tov3

Change-Id: I7d2f006ef6dbbcd6b41c8adbeee0c0e048d35448
Closes-Bug: #1281342
Merge "Fix element miss in client request body"