Port server_usage API to v3 part 1

This changeset only copies the v2 files 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: Ie5cbde7d9a0d77e78c9f9cc577d8cbd36fb74188
Merge "Port server_usage API to v3 part 1"