When running testr outside of venv use full path.

* modules/jenkins/files/slave_scripts/run-tox.sh: When running testr
from outside of the venv provide a full path to testr. Testr cannot be
found otherwise.

Change-Id: I25602847d389d4ddc62acd872e54727d6f309443
Reviewed-on: https://review.openstack.org/26783
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins