Update launch script and instructions

* launch/README: Correct outdated references to salt and puppet
certs. Modernize the recommended image name.

* launch/launch-node.py: When run as root, keep the puppet group on
private keys so that they're readable by puppet group members. That
way it won't break subsequent runs as non-root following the current
instructions in the README file. Modernize the default image name.

Change-Id: Ic5034121d4d0df3c096c2e474a9d3d41ec361021
Merge "Update launch script and instructions"