Bump default Auto DevOps deploy image to 2.18.1

What does this MR do and why?

Bumps default the Auto DevOps auto-deploy-image to from 2.17.0 to 2.18.1.

Most importantly, this new version includes a bug fix for the agent to work with the Auto Deploy feature of Auto DevOps. See related issue: gitlab-org/cluster-integration/auto-deploy-image#191 (closed)

Manual QA

The new bumped version was tested here: https://gitlab.com/Alexand/min-ruby-app/-/jobs/2020053269

The image used was customized here: https://gitlab.com/Alexand/min-ruby-app/-/blob/master/.gitlab-ci.yml#L23

How to set up and validate locally

1. Make sure you have a cluster with Ingress installed
2. Fork the https://gitlab.com/Alexand/min-ruby-app
3. Make sure you Register the agent and install it. The folder for the registration is already prepped on `.gitlab/agents/my-agent/config.yaml
4. Override the KUBE_INGRESS_BASE_DOMAIN in the .gitlab-ci.yml with your base domain as expected for Auto DevOps.
5. Set the KUBE_CONTEXT in the .gitlab-ci.yml to <namespace>/min-ruby-app:my-agent.
6. Trigger an Auto DevOps pipeline. The deploy should succeed.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.