Convert multiple-create v3 plugin to v2.1

Changes required to have the multiple create v3 plugin natively
support the v2.1 API. The following Server creation parameters
are reverted:

os-multiple-create:min_count -> min_count
os-multiple-create:max_count -> max_count
os-multiple-create:return_reservation_id -> return_reservation_id

Removes min_count parameter for a test which should never have
been included but due to a gap in input validation was ignored.

Partially implements blueprint v2-on-v3-api

Change-Id: Ie435c357958066e9fde40eab54826261ee9d4226
Merge "Convert multiple-create v3 plugin to v2.1"