tox: remove sitepackage=True
tox: remove sitepackage=True

remove sitepackage=True to set sitepackage=False by default.
sitepackage=True causes pip install error with stable/ocata branch.
appdirs 1.4.3 is installed on system, but stable/ocata requires appdirs 1.4.0.
pip find appdirs 1.4.3 of global package, but pip fails to uninstall appdirs
in venv.
sitepackage=False resolves it.

Change-Id: I40b60f53b18b60594f9adcf1e280b2b32d35d123
Closes-bug: #1671323

openstack/networking-odl,openstack/networking-odl,openstack/networking-odl