tox: only install all storage drivers in py-$index or py-#index-all
tox: only install all storage drivers in py-$index or py-#index-all

There's no need to install e.g. Ceph when testing with Swift.

Change-Id: I302f3fe4cd8e886e964961ece31228fa349ee797

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