Cleanup tox.ini constraint handling

Use the "modern" way of contraints setup and remove double setting of
constraints (in install_command and deps) for some environments.

Remove also -U from pip install command, it can break with constraints
and update required packages.

Change-Id: I2412a02dcba40a3128f9af766e27c046ce3d3f25

tox.ini

@@ -5,11 +5,12 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -U -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv =
     VIRTUAL_ENV={envdir}
     DISCOVER_DIRECTORY=sahara/tests/unit
 deps =
+    -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
     -r{toxinidir}/requirements.txt
     -r{toxinidir}/test-requirements.txt
 commands = stestr run {posargs}