Port flavormanage extension to v3 API Part 2

Ports the flavormanage extension and the corresponding
unittests to the v3 framework.

Partially implements blueprint nova-v3-api

Change-Id: Iaf6b4a2d7f4e4993a6540e8a3afc774b7b158608
Merge "Port flavormanage extension to v3 API Part 2"