Implements triggering individual deployments to staging and cny/prod.
These deployments are triggered using the 'next-gen' branch (dry-run
mode) on Deployer.

Implementation changes summarized:

* Transforms AutoDeploy::Deploy (now AutoDeploy::DeployTrigger) in a
  generic class that takes two arguments:
  * environment: The environment to deploy to
  * ref: The deployer branch to use to
* Execute this new class through rake tasks
* Create a new/generic class to wait for a pipeline to complete
* Modify gitlab-ci.yml with the new builds to trigger a deployment to
  staging, wait for it, and then trigger a deployment to canary.

Deployment to production is triggered during the canary deployment. In
the next iterations, an individual deployment will be triggered for this
environment.

Related to gitlab-com/gl-infra/delivery#1578