Description of problem:
When build env by ansible, the docker-registry's config: config.yml  not contain the option: REGISTRY_MIDDLEWARE_REPOSITORY_OPENSHIFT_MIRRORPULLTHROUGH .
The docker-registry's image contain that option by default.

Version-Release number of selected component (if applicable):
openshift version
openshift v3.5.0.17+c55cf2b
kubernetes v1.5.2+43a9be4
etcd 3.1.0

How reproducible:
always

Steps to Reproduce:
1. Build OCP3.5 env by ansible;
2. Check the registry's config:
oc exec -it docker-registry-2-drj5n more /etc/registry/config.yml
version: 0.1
log:
  level: debug
http:
  addr: :5000
storage:
  delete:
    enabled: true
  cache:
    blobdescriptor: inmemory
  s3:
    accesskey: xxxxx
    secretkey: yyyyy
    region: us-east-1
    bucket: bucket1
    encrypt: false
    secure: true
    v4auth: true
    rootdirectory: /registry
    chunksize: "26214400"
auth:
  openshift:
    realm: openshift
middleware:  
  registry:  
  - name: openshift
  repository:
  - name: openshift
    options: 
      pullthrough: True
      acceptschema2: False
      enforcequota: False
  storage:   
  - name: openshift


Actual results:
2. The default config not contain option: REGISTRY_MIDDLEWARE_REPOSITORY_OPENSHIFT_MIRRORPULLTHROUGH

Expected results:
2. Should contain option: REGISTRY_MIDDLEWARE_REPOSITORY_OPENSHIFT_MIRRORPULLTHROUGH

Additional info: