[fluentd-elasticsearch] Provide option to disable settings elasticsearch hosts

[ArmaanT]

What this PR does / why we need it:

This PR provides a way to disable setting the OUTPUT_HOSTS environment variable from the elasticsearch.hosts value. This feature allows users to manually set OUTPUT_HOSTS using a secret or alternative method if they desire.

Which issue this PR fixes

• fixes #348

Special notes for your reviewer:

N/A

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• [x] DCO signed
• [x] Chart Version bumped (if the pr is an update to an existing chart)
• [x] Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [fluentd-elasticsearch])

[ArmaanT]

@monotek I changed the name over to useOutputHostEnvVar, but I think something like setOutputHostEnvVar may make more sense, because then a false value means the chart won't set that environment variable.