Convert v3 servers return_reservation_id behaviour to v2.1

Changes return reservation id format on server create to
the v2.1 format.

Adds api sample tests for the os-multiple-create plugin

Partially implements blueprint v2-on-v3-api

Change-Id: I148cce80fb4b19a4240e00ca206fcb7bac20f357
Merge "Convert v3 servers return_reservation_id behaviour to v2.1"