It is an elasticsearch error, but I'm not sure it's unrelated to Ansible. Reading the elasticsearch source, it looks like elasticsearch looks for a resource to load called elasticsearch.yml. It looks in a few places, but my suspicion is that it finds this file in $PATH. So does Ansible add playbooks to $PATH?
Elasticsearch will look in the current working directory for elasticsearch.yml. However, I'm skeptical that this is the reason because I'm running a script that loads the playbooks and calls ansible-playbook on them. When I'm running my script, I'm not running it from the same directory where the playbooks are located. And my script doesn't cd into the playbooks directory or add the playbooks directory or any playbooks to $PATH. So unless, Ansible cd's into the playbooks directory, the $PATH scenario is more likely. It's entirely possible, but I have my doubts that Ansible is changing directories since it doesn't seem necessary.