Run neutron functional job as tempest user

Previously, invoking tox on the cloned neutron repo was
resulting in a permission error because the default user did
not have permission to write to /opt/stack/new/neutron.  This
change ensures the tempest user can write to that path and
is the one that executes tox.

Change-Id: I18fbaf5fc0acc85dd0b39ae26c008e634c118730
Merge "Run neutron functional job as tempest user"