Fix bug where scaling of replicas fails

Auto DevOps uses --reuse-values which means that the chart's default values.yaml does not get applied. See https://github.com/helm/helm/issues/3957#issuecomment-384685681.

To resolve this we need to introduce defaults in the chart as well.

Resolves #48 (closed)

To test this:

1. set PRODUCTION_REPLICAS to 2
2. In Settings >> CI/CD >> Auto DevOps, select Automatic deployment to staging, manual deployment to production, as deployment strategy
3. Run an Auto DevOps pipeline, manually triggering rollout 50%, and rollout 100%.
4. Run another Auto DevOps pipeline, manually triggering rollout 50%, and rollout 100%.

• ❌ failed on v0.5.0 => https://staging.gitlab.com/hello-staging/minimal-ruby-app/-/jobs/37088208
• ✅ passed on chart packaged from this branch => https://staging.gitlab.com/hello-staging/minimal-ruby-app/-/jobs/37088220