Even more configuration for elasticsearch

[shreddedbacon]

Checklist

• Affected Issues have been mentioned in the Closing issues section
• Documentation has been written/updated.
• Changelog entry has been written

This is additional to PR #1182 and allows even more configuration to be specified via environment variables, but allows for free options rather than specific types.

Options specified as a single variable separated with newline values

export EXTRA_OPTS="processors: 4\ntransport.netty.worker_count: 4\nxpack.watcher.thread_pool.size: 20\nthread_pool.fetch_shard_started.max: 8\n"

which results in the following being added to config/elasticsearch.yml during container start up

processors: 4
transport.netty.worker_count: 4
xpack.watcher.thread_pool.size: 20
thread_pool.fetch_shard_started.max: 8

Changelog Entry

Improvement - Allow any additional configurations to be added to elasticsearch

Closing issues

closes #1181