1. Proposed title of this feature request
PaC: Expand “dynamic” variables in pipeline default values

2. What is the nature and description of the request?
When adding an expandable dynamic variable as a default value to a parameter in a Pipeline, it is not resolved.

Pipeline.yaml:

- name: REPO_NAME
    default: "{{ repo_name }}"
  - name: REVISION
  - name: BRANCH
  - name: PULL_REQUEST_NUMBER

Omitting the param with a default value in the PipelineRun does not resolve the params.

The following error is thrown:

http.client.InvalidURL: URL can't contain control characters. '/repos/< organisation>/{{ repo_name }}/statuses/< sha>' (found at least ' ')
This error should not be thrown, but dynamic variables should be expanded in the pipeline spec, making it possible to use them as default values. 

3. Why does the customer need this? (List the business requirements here)
This would severely clean up pipelines as it is not needed to pass through all expandable dynamic parameters. In effect, it would make shared pipelines easier to use, and reduce the surface to make mistakes.

4. List any affected packages or components.
OpenShift Pipelines