Add missing logstash environment on documentation

[thyarles]

Description

Update documentation to add missing logstash environment password.

Issues Resolved

https://github.com/opensearch-project/ansible-playbook/issues/134

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[castorsky]

I miss this in documentaion too. Had to dig through role to find out why cluster did not start.

[bbarani]

@saravanan30erd @prudhvigodithi @peterzhuamazon Can you please review this PR?

[prudhvigodithi]

Thanks @thyarles, if this is useful, please backport to 1.x branch as well.