Design Artifact: Inform user of unfinished steps when setting up Auto DevOps

If a user enables Auto DevOps and does nothing else, then Auto DevOps will not work properly. We need to inform the user that a domain is required. We also need to inform the user about creating a cluster. This could be done with a flash message, similar to how we tell users they need to add an SSH key.

Unfinished steps could include:

• Not setting up a domain
• Not setting up a cluster
• Not installing necessary applications to your cluster
• Not setting up external IP

This is directly related to https://gitlab.com/gitlab-org/gitlab-ce/issues/43466 and https://gitlab.com/gitlab-org/gitlab-ce/issues/43467.