This change imports (with slight modification) the
MicroversionMiddleware created for the placement service.
For the time being the middleware relies on WebOb as that is
what it originally relied on. If there are requirements to do
otherwise, later, we can make adjustments then.
The README is updated with limited usage instructions.
Gabbi tests are added, requiring adjustments to the test configuration,
so the opportunity was taken to switch to using stestr instead of testr.
Though arguably "functional" tests because they are not strictly unit
tests, no distinction is made in the tests directory, for now, as it
is all nice and fast and we'd like to keep it that way.