fix(helpers) disable keyring in wait-for-db container

[fffonion]

What this PR does / why we need it:

The wait-for-db container checks for migration readiness. Since
it starts the full kong cluster, it also runs keyring bootstrap process:
the kong clster in that container generates the initial key,
store in its memory, and writes the key_id to keyring_meta table.
But it doesn’t have enough time to broadcast its key to other kong
container that serves proxy, admin, manager etc. By design,
keyring doesn't automatically generate a new key if there's
anything in the keyring_meta table, even no nodes have
knowledge to it; this it to avoid unnecessary rotation of keys.

This PR skips keyring bootstrap at wait-for-db container, so the
bootstrap can be done in an actual kong cluster.

Which issue this PR fixes

Special notes for your reviewer:

Checklist

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

• PR is based off the current tip of the main branch.
• Changes are documented under the "Unreleased" header in CHANGELOG.md
• New or modified sections of values.yaml are documented in the README.md
• Commits follow the Kong commit message guidelines

[CLAassistant]

All committers have signed the CLA.

[fffonion]

Thanks for the review @mflendrich, I have addressed the comments, could you take another look : )?