Port used limits extension to v3 API Part 1

This changeset only copies the v2 implementation file into the
appropriate v3 directory 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: I34214370f53d481346eb11fec6b8fd0ddeb42918
Merge "Port used limits extension to v3 API Part 1"