Disable cron poller via envvar

What does this MR do and why?

This MR allows users to disable the cron_poller by setting the environment variable GITLAB_CRON_JOBS_POLL_INTERVAL=0. This functionality will be used by gitlab.com to deprecate the use of namespace in Sidekiq (See details of per-shard rollout gitlab-com/gl-infra/scalability#2481).

We can disable the cron poller by setting cron_poll_interval to a negative number. However, configMap configures cron_job via the global.appConfig map which prevents us from targeting individual shard deployments.

A targeted approach via envvar allows us to run 2 groups of sidekiq workers with different SIDEKIQ_ENQUEUE_NON_NAMESPACED values (!116379 (merged)). This controlled rollout is safer than a big-bang approach of disabling all namespaces in all Sidekiq pods on 1 deployment.

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Before	After

How to set up and validate locally

On the master branch, visit http://gdk.test:3000/admin/sidekiq/cron. The last enqueued column will update over time (set live polling on the top right corner).

On this branch

1. Set GITLAB_CRON_JOBS_POLL_INTERVAL=0 in your gdk's env.runit file.
2. Restart sidekiq via gdk restart rails-background-jobs
3. Monitor the cron page http://gdk.test:3000/admin/sidekiq/cron, no more crons should run. tip: just leave it for 5-10 minutes, there should be no more just now or x seconds ago.

MR acceptance checklist

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