tox: create a target for each indexer
tox: create a target for each indexer

Change-Id: Ifaab92b607cf5280a9b8556b6f9764c5b8533c01

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