elasticsearch-ephemeral: Disable automatic creation of indices

[smatting]

This PR disables the automatic creation of indices in the elasticsearch-ephemeral chart.

Without this PR ES will create an index whenever a document is attempted to be indexed. This leads to a race-condition right after the elasticsearch-ephemeral release is deployed and all indices have been removed:

A) brig or brig-index-migrate-data indexes a user into a newly auto-created index directory (with an incorrect mapping)
B) The elasticsearch-index-create job creates a index directory (with the correct mapping) if it doesn't exist yet

If A) happens before B) the directory index is useless to brig. Brig will not be able to find any users, because it assumes that fields like team are of type keyword. The auto-created index however gives all fields the type text.
If B) happens before A) all is good.

This PR assures that A) cannot happen before B), because auto-creation is disabled.

Checklist

• The PR Title explains the impact of the change.
• The PR description provides context as to why the change should occur and what the code contributes to that effect. This could also be a link to a JIRA ticket or a Github issue, if there is one.

[jschaul]

🎆 Looks good, saw this working on screen share 😸