Use variables to define deploy policy for canary

Note: This issue requires https://gitlab.com/gitlab-org/gitlab-ce/issues/37397.

Description

The current Auto DevOps template already contains support for canary deployments, but this is disabled by default and if you want to enable it you need to make the template explicit. We should allow to define a group/project variable to define if you want to use canaries for production deployments.

Proposal

Improve the Auto DevOps template in order to support canaries for deployments.

Required changes to the canary job:

1. always defined (no trailing . in the job name)
2. enable only when $CANARY_ENABLED