You'll get to use a lot of quotes in your puppet manifest; I _think_
something along the lines of
augeas { foo:
context => ...,
changes => "set ETHTOOL_OPTS \"'autoneg on'\""
}
should do the trick. The outer double quotes are stripped by puppet when
it reads the manifest, the inner double quotes are stripped by the
augeas type when it parses the changes, leaving the single quotes in the
value that gets passed to augeas.
David