* Only tox has to be installed via pip, everything else is in virtual
environment. Nose is no longer used.
* One should NOT run other tox commands in an existing tox environment.
This is not how tox is intended to be used, and it cause weird failures
or seemingly random virtual environment rebuilds.
* Drop statement about running unit tests on a clear VM. Not sure where it
comes from, only tox and setuptools are required to run unit tests,
and they don't mess with system packages. Only leave note about devstack,
as people seem to have problems with it.
Change-Id: Ie7d0172b66231cfd8f9c4f8f2d76604a187443b0
3ce0401da0