Port multiple_create extension to V3 API

Ports the multiple create extension to the V3 API. There is no part1
patch for this changeset because the original V2 file was just a stub
with all of the code in servers.py

DocImpact
Partially implements blueprint nova-v3-api

Change-Id: I5eba468e9c4b753215b91607a3e2441626d2cb57
Merge "Port multiple_create extension to V3 API"