Use upper-constraints in tox runs

The upper-constraints file wasn't being enforced
when installing dependencies in tox runs which can
lead to failures like installing Sphinx>2.0 in py27
where it's not supported.

This is loosely based on novaclient change
I8be883215f27abb58d15b85e8542cbdf32000bac for the
same kind of issue.

Note there are other things that should be done
in here regarding requirements cleanup, like the
sphinx dependnecies should move to a
doc/requirements.txt file to separate them from
unit test runs, but that can be dealt with in a
follow up change.

Change-Id: I359a35af5b653299110a8d0faddb255da0b10118
Closes-Bug: #1828698

tox.ini

@@ -5,12 +5,15 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command =
-                  constraints: pip install -U --force-reinstall -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt} {opts} {packages}
-                  pip install -U {opts} {packages}
+passenv = ZUUL_CACHE_DIR
+          REQUIREMENTS_PIP_LOCATION
+install_command = pip install {opts} {packages}
 setenv =
    VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/test-requirements.txt
+deps =
+  -c{env:UPPER_CONSTRAINTS_FILE:https://opendev.org/openstack/requirements/raw/branch/master/upper-constraints.txt}
+  -r{toxinidir}/test-requirements.txt
+  -r{toxinidir}/requirements.txt
 commands = rm -f .testrepository/times.dbm
            stestr --test-path=./watcherclient/tests/unit run {posargs}
            stestr slowest