Use the testr from os-testr env
Use the testr from os-testr env

The functional tests have site-packages set to true for the tox
environment to make some of the live tests work correctly. In the
recent past this has meant that testr is now no longer available in
.tox/py27-gate/bin. Instead we used one that we know will be there.

Change-Id: I77befff673f46de85510d60cc4faf1c85d36086d

sileht/gnocchi,leandroreox/gnocchi,sileht/gnocchi,leandroreox/gnocchi,gnocchixyz/gnocchi,gnocchixyz/gnocchi