Add database config sanity check to gitlab-rake gitlab:check

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We recently dealt with an issue where a user's external Postgres database configuration causing the same pipelines to be triggered multiple times. In this particular case, they have default_transaction_isolation configured as repeatable read instead of read committed (internal only ZD ref: https://gitlab.zendesk.com/agent/tickets/119156)

I've documented the troubleshooting error in our documentation here: omnibus-gitlab!3359 (diffs)

It would be great if our gitlab:check can catch these kinds of potential database configuration issues.