Port flavor_rxtx extension to v3 API Part 1

This changeset only copies the v2 files (implementation and test)
into the appropriate v3 directories unchanged. The copy as-is
will not be loaded by either the v2 or v3 extension loaders. The
second changeset will then make the changes required for it to
work as a v3 extension.

This is being done in order to make reviewing of extension porting
easier as gerrit will display only what is actually changed for v3
rather than entirely new files

Partially implements blueprint nova-v3-api

Change-Id: I560aacfcde07481c2bcc6d8ab4c785ba6e2c2e2b
Merge "Port flavor_rxtx extension to v3 API Part 1"