Add a periodic job to run the stress tests.

This commit adds a periodic jenkins job to run the tempest
stress tests.

Change-Id: I58edb56d19c3b4e3b572e02b6ece42955a4d9933
Reviewed-on: https://review.openstack.org/35397
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins