Test for IPv6 DSCP related issues.

* launch/launch-node.py: Sometimes we get a broken server from a
provider, probably actually a broken compute node behind the scenes,
which fails to send IPv6 packets with a DSCP > 3. Test that we can
send DSCP 0x10 packets to at least one IPv6-connected host and get a
response, or else discard the server.

Change-Id: I379589d74f9e5892e3fda85bd267567038b6c2c9
Merge "Test for IPv6 DSCP related issues."