Increase example Jenkins slave flavor.

* launch/README: The preferred flavor for a Jenkins slave in
Rackspace nova has RAM 8192, so we can get the same 4 CPU count we
had with Rackspace legacy VMs.

Change-Id: I55563ff966061b5c02fb68b671156d782c2d042d
Reviewed-on: https://review.openstack.org/23658
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