Fix post jobs

The tox venv environment is run during post jobs and thus cannot use
constraints.

See
http://logs.openstack.org/a6/a6650650f065bc4423568f9318d362957eba7f31/post/magnum-docs/60418d4/
http://logs.openstack.org/a6/a6650650f065bc4423568f9318d362957eba7f31/post/magnum-branch-tarball/415035c/

We run for docs jobs:
"tox -e venv python setup.py build_sphinx"

thus, the docs environment is not used.

For branch tarball, the infra scripts use:
"tox -e venv python setup.py sdist"

And infra does not setup constraints for post jobs currently as this is
not working with current tools

Fix tox.ini for this.

Change-Id: Iad81b47899c4554b75f90dc24f0aa2aff7aef730

tox.ini

@@ -84,6 +84,10 @@ commands =
   bandit -r magnum -x tests -n5 -ll
 
 [testenv:venv]
+# NOTE(jaegerandi): This target does not use constraints because
+# upstream infra does not yet support it. Once that's fixed, we can
+# drop the install_command.
+install_command = pip install -U --force-reinstall {opts} {packages}
 commands = {posargs}
 
 [testenv:venv-constraints]