Add three more logstash workers nodes.

We keep adding more tempest tests to run per patchset. These tests are
the most expensive to index as they come with many megabytes of
uncompressed logs. Add more logstash worker nodes in an attempt to keep
up.

Change-Id: I41d35792d58d1b0456927b0f5b1b7728bf3d145c
Merge "Add three more logstash workers nodes."