Auto DevOps should check if a release exists before delete

Description

Auto DevOps template tries to delete canaries even if they are not there. This creates an error in the log, that can be misleading for users.

$ delete canary
Error: release: "production-canary" not found
Job succeeded

We should avoid this to happen checking in advance if the release is there. Since it is handled in the helper function delete() (https://gitlab.com/gitlab-org/gitlab-ci-yml/blob/master/Auto-DevOps.gitlab-ci.yml#L397), changing it will automatically improve all similar cases.