Index logs/screen-n-api-meta.txt for logstash queries

We are seeing a race failure in check-tempest-dsvm-postgres-full jobs
where the error message shows up in the screen-n-api-meta log but we can
only get at it from screen-n-api logs (or console). We should index on
the screen-n-api-meta log so we can better filter the query.

Related-Bug: #921858

Change-Id: I1701ac83b2643d819245a7cdbbfb56cc4af12f5b