One more update: I used the log4j2.properties file from Elastiscsearch 1.7.3 on version 5.2.0, and logging ran.
This turns out to be about the version of the elastic/elasticsearch Puppet module used. We're using release 6.0.0, which is the first version of the module that changed the variables used in log4j2.properties. It seems that I might want to consider backing off to release 5.5.0, which has the right variables in use in that erb file.