Have tox install via setup.py develop

tox 1.6 was released, which means that we can now take advantage of
the feature we added to it - which is using setup.py develop to
install the code into the virtualenv. The logic was taken from
run_tests.sh - so the performance issues around using tox vs. using
install_venv should now be gone.

Additionally, override the tox pip install command to avoid using
the "--pre" option which is the default in tox. "--pre" means
"Include pre-release and development versions." By default, pip will
only install stable versions of software, and that is the behavior
we want.

Change-Id: Ida5e440d1bdb9f8e9031277ea53a02d2ef171438
Co-Authored-By: Monty Taylor <mordred@inaugust.com>
Update to latest pbr

This commit moves switches tempest from the d2to1 version of pbr and
move to the latest version of pbr. However, there is a bug with
build_spinx so it doesn't work with a doc version set so that section
is removed from the doc conf.py.

Change-Id: I4fe14eb2b2e81132f8fa99936a031176b38c437b
(cherry picked from commit 271139aa7f1fa5f8d9e663702bf275378456fa46)
Updated from global requirements

Change-Id: I4b81481201d859679585d1885fa478ecc5238042
Merge "Have tox install via setup.py develop" into stable/havana