elastic-workers: Let the loader some time to finish gracefully

Related to swh/infra/sysadm-environment#4144 (closed)

• [1] https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes/#probe-level-terminationgraceperiodseconds

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Migrated from D7809 (view on Phabricator)