Trigger individual deployments to staging and cny

What does this MR do?

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 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 (closed)

CI pipeline

Note: Image taken from the CI editor

To do

• Add a class to trigger a staging deployment
• Add class to trigger cny/gprd deployment
• Modify .gitlab-ci.yml to trigger individual deployments
• Add rake tasks
• Make the code pretty