Fix post install jobs

The venv environment is used for post install jobs and thus cannot
currently use constraints, use an unconstraint install command for it.

Example failure:
http://logs.openstack.org/b8/b8a70e4aeca83ebefad4b127af71b1bd125efa40/post/zaqar-branch-tarball/d9fe983/

Change-Id: If6e9f85b3a8b15048453bf1177812fb37a8e2d54

tox.ini

@@ -40,6 +40,10 @@ commands =
     --testr-args='^(?!.*test.*coverage).*$'
 
 [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:docs]