Tell pip to install packages it sees globally

Use pip's option to force installation of packages so that
local copies are installed in the virtualenv, even if they
are present in the global site-packages directory. This
ensures that the latest packages are installed, and that
all of the oslo libraries from namespace packages are
installed in a consistent way, making it possible to run
unit tests on a system where devstack was run previously.

Change-Id: If5c4e12956f2c7be6530c5c9a57fdfb1da8e710c

tox.ini

@@ -6,7 +6,7 @@ skipsdist = True
 [testenv]
 sitepackages = True
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command = pip install -U --force-reinstall {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt