Description of problem:
Deployment hook pull image will fail with old dockercfg scheme:
{"https://index.docker.io/v1/":{"username":"wjiang","password":"kkkkkkkk","email":"wjiang","auth":"xxxxxxxx"}}
But succeed with new dockercfg scheme:
{
        "auths": {
                "https://index.docker.io/v1/": {
                        "auth": "uuuuuuuuu",
                        "email": "wjiang"
                }
        }
}

Version-Release number of selected component (if applicable):
devenv_rhel7_2638

How reproducible:
Always

Steps to Reproduce:
1. Create template app via https://raw.githubusercontent.com/openshift/origin/master/examples/sample-app/application-template-stibuild.json
2.Generate a secret via `oc secrets new-dockercfg`
oc secrets new-dockercfg docker --docker-username=wjiang --docker-password=qqqqqqq --docker-email=wjiang
3.Modify the deploymentConfig frontend to use dockerhub private repo  with both ImagePullSecrets and hooks
strategy:
    resources: {}
    rollingParams:
      intervalSeconds: 1
      maxSurge: 25%
      maxUnavailable: 25%
      post:
        execNewPod:
          command:
          - /bin/false
          containerName: ruby-helloworld
          env:
          - name: CUSTOM_VAR2
            value: custom_value2
        failurePolicy: Ignore
      pre:
        execNewPod:
          command:
          - /bin/true
          containerName: ruby-helloworld
          env:
          - name: CUSTOM_VAR1
            value: custom_value1
        failurePolicy: Abort
      timeoutSeconds: 120
      updatePeriodSeconds: 1
    type: Rolling
.................................................
        image: wjiang/origin-ruby-sample
        imagePullPolicy: Always
        name: ruby-helloworld
        ports:
        - containerPort: 8080
          protocol: TCP
        resources: {}
        securityContext:
          capabilities: {}
          privileged: false
        terminationMessagePath: /dev/termination-log
      dnsPolicy: ClusterFirst
      imagePullSecrets:
      - name: hubwjiang
      restartPolicy: Always
  triggers:
  - type: ConfigChange
4. Try to deploy frontend if not trigger automatically
5. Replace old dockercfg scheme with new
6. Repeat step 4

Actual results:
4. Pull private image failed
6. Pull private image succeeded

Expected results:
4. Pull private image succeeded

Additional info: