Fix where we use constraints

in I2598503c2d4397d7efbb03a76df9a4e91dd0c1cf we mistakenly added
constraints to install_command in out based testenv.  This is incorrect
as it means the generate env will be run in a constrained environment.

We don't use UPPER_CONSTRAINT_FILE as *right now* it's the same thing
and this leaves us internally consistent (even if technically incorrect)

Change-Id: Ia61d944dbfdf494598e41e5fbefd0e32a91814ed

tox.ini

@@ -7,9 +7,10 @@ ignore_basepython_conflict=true
 [testenv]
 basepython = python3
 usedevelop = True
-install_command = pip install -U {opts} -c {toxinidir}/upper-constraints.txt {packages}
+install_command = pip install -U {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
-deps = -r{toxinidir}/requirements.txt
+deps = -c{toxinidir}/upper-constraints.txt
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands =
   stestr run {posargs}