Remove link to devstack_launch_slave.pp

The devstack_launch_slave.pp module was removed on 2013-08-18.  This
change updates the documentation to reflect that deletion.

Change-Id: I3ae522cfbdc4e3650e4921fb633b21192f0fee85
Closes-Bug: 1325379
Merge "Remove link to devstack_launch_slave.pp"