More launch improvements

* launch/README: On servers where the puppet agent daemon needs to
enabled, don't run the initscript directly under sudo. Passing it
through su avoids permission issues with the puppet mysql module.

* launch/launch-node.py: Only test IPv6 connectivity when providers
support it.

Change-Id: Ic37305331ba57ba3de2fb349e1ab503cabcc697d
Merge "More launch improvements"