Set a sane default of 2 seconds for Unicorn 'readinessProbe.timeoutSeconds'

This is a first step towards #1576.

I observed gitlab-workhorse containers in a Ready=false state (e.g. https://console.cloud.google.com/kubernetes/deployment/us-central1-b/review-apps-ee/review-apps-ee/review-use-merge-0gi02b-unicorn?project=gitlab-review-apps), and I checked manually and the /scripts/healthcheck usually takes more than 1 second to run.