Set dsvm-functional job to use system packages

The functional job was breaking due to the interaction between
devstack installing neutron system-wide in editable mode (pip install
-e) and tox packaging in the same path.  Installing in editable
mode meant that neutron.egg-info/PKG_INFO could be updated (in this
case by tox) to change the installed version of the neutron package
without updating scripts that depended on the installed version
(e.g. rootwrap).  This fix is to set the dsvm-functional env to
use system packages and avoid having tox update PKG_INFO.

Change-Id: Icc38cacd69bc5843ccfcc60237a7102df6d8597f
Closes-bug: #1370347