Add reference keyword to CI service definition

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What does this MR do and why?

In CI service definition command can use reference keyword

Add capability to do this:

.dind-registry-mirror:
  command:
    - "--registry-mirror"
    - "https://mirror.gcr.io"

job_name:
  services:
    - name: docker:dind
      command: 
        - !reference [.dind-registry-mirror, command]
        - "--tls=false"

Signed-off-by: guillaume.micouin-jorda gmicouin@netcourrier.com

Screenshots or screen recordings

Before	After

Issue: #517479 (closed)