Pass Jenkins' SSH pub key to special slaves.

The openstack_project::slave refactor defaulted the Jenkins' SSH pub key
to an empty string. This erased the authorized keys file for the Jenkins
user on our specialized slaves. Restore that file's contents.

Change-Id: I28067017ec2dc36feef9df076099813abf820ee9
Reviewed-on: https://review.openstack.org/31419
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Tested-by: Jenkins