CI/CD - Add image promotion pipelines
    
    This PR comes with an pipeine template from which users can instantiate image promotion pipelines to promote image
    between environments.
    Env files for instantiate pipeline jobs for promoting :dev to :stage and
    :stage to :prod are also included.
    
    A specially designed test suite should be run to ensure the image is mature
    enough before promotion. Since we currently don't have such a test suite,
    this pipeline still need to be manually triggered.
    
    What this OpenShift Pipeline does is basically:
    - Pulling the image to be promoted
    - Pushing the image to destinations with promoted tags.
    - Optionally tagging the promoted image into an image stream after pushes.
    
    NOTE:
    1. This pipeline *DOES NOT* cover the step of running tests before actually promoting the image.
    2. It's designed to be a callback pipeline triggered by a microservice that handles Greenwave messages.
    3. It can be triggered manually (as describe below) to force promoting an image without any tests.
    
    Please refer to `openshift/README.md` for instructions.