The development.environment instructions were almost completely copy/pastable
to get started on a new machine. Almost. This doc patch just adds python-tox to
the list of things to be fetched with apt-get, so now it's really *really* simple to get
started on a brand-new machine or cloud instance.
Change-Id: I09cbb376df2ce81e618c99e5db36ddf7809f6f3d
e2c5aef530