Port flavormanage extension to v3 API part 1

This is part 1 of the two part commit to port migrations to v3.
As part of this commit the api and test files are copied over to v3
plugins folder. The second changeset will have changes to port
migrations to v3.

Partially implements blueprint nova-v3-api

Change-Id: I2a51185f557c68022a2f3aaa006acb1e89b3c737
Merge "Port flavormanage extension to v3 API part 1"