Add jenkins ssh private key to devstack-launch.

It's needed to perform the test-logins to the new servers.

Change-Id: I8fd5cea8dd1b807b42acf508fcd4853bb735150e
Reviewed-on: https://review.openstack.org/24497
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Elizabeth Krumbach <lyz@princessleia.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins