Update launch docs for salt permissions.

* launch/README: Mention adding yourself to the salt group.

* modules/salt/manifests/master.pp: Loosen directory permissions
minimally as needed for salt group members to be able to run the
launch script without being root.

Change-Id: I4e462fe2efabe2200a635c79e4b7a1314bf174a3
Reviewed-on: https://review.openstack.org/27562
Reviewed-by: Jesse Keating <jesse.keating@rackspace.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins