tox: factorize docs dependencies
tox: factorize docs dependencies

This has been fixed in tox 2.4.

Change-Id: I040bd1a12ade7a255e0d52c2c1fb6ff61be007d9

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