Problem: Elasticsearch 5.x fails when using thread_pool.write.* default options

[mamedin]

After merging #19, when installing elasticsearch 5 with default options it fails because the following options don't exist in ES 5, they are ES 6 variables:

https://github.com/artefactual-labs/ansible-elasticsearch/pull/19/files#diff-7eeda618087b49ae876084ab6c73fdbbR41-R42

ES 5 must use:

- "thread_pool.bulk.size: 2"
- "thread_pool.bulk.queue_size: 1000"