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
Port used limits extension to v3 API Part 2

Ports the used limits extension and the corresponding unittests to
the v3 framework.

Partially implements blueprint nova-v3-api

Change-Id: I6f1aa507ffd297e56d9cfd23972bc3bb15ba181a
Merge "Port used limits extension to v3 API Part 1"
Merge "Port used limits extension to v3 API Part 2"
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp