[Feature/multi_tenancy] Make tenant awareness setting static

[dbwiddis]

Description

Removes the ability to change the value of the mutiTenancy setting: it must be initialized in opensearch.yml.

Two other small changes:

• When multiTenancy was false (which is was while I was figuring out how to set it), Remote client search didn't include the index name, thus returning every document in the cluster/collection.
• improved performance of removing the dot from a system index name from a match-all regex to substring, since I briefly thought this could be a problem when debugging the bug above

Check List

• [x] New functionality includes testing.
• [x] Commits are signed per the DCO using --signoff.

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