add nox support (but dont switch out fully)

add nox support (but dont switch out fully)

This backports the nox change from main/2.1 so that we have full
nox support available for the 2.0 series, however does not modify
any existing docs or test systems.

A noxfile.py has been added to allow testing with nox.  This is a direct
port of 2.1's move to nox, however leaves the tox.ini file in place and
retains all test documentation in terms of tox.   Version 2.1 will move to
nox fully, including deprecation warnings for tox and new testing
documentation.

Change-Id: I66639991e1dc3db582e2ff13f9348a7d6241916e
(cherry picked from commit df899e94cf7ba18f4e7151ef173393be78c56c3f)
(also cherry-picked from 1577c1d15b)