Use consistent naming for ca_file/cacert parameters across Elasticsearch input/output plugins

[joshuar]

In this output, we use the parameter cacert for specifying a path to a PEM encoded file holding the CA cert for an SSL-secured ES cluster while we use ca_file in the output plugin. As these are the same parameters, they should have the same name.

See also logstash-plugins/logstash-input-elasticsearch#42