correspondence with 'logstash_prefix flog' in 'kolla-ansible/ansible/
/roles/common/templates/conf/output/01-es.conf.j2'
and 'kolla-ansible/ansible/roles/common/templates/conf/output/
/00-local.conf.j2'.

Change-Id: I90a7b1b6a5267d79f192345e531b503d3a99ca79