Bug 1555165

Description of problem:
While trying to sync google container registry from satellie 6.3 GUI, we get following error:

Output
Could not find registry API at https://gcr.io
Errors:
DKR1008: Could not find registry API at https://gcr.io

Version-Release number of selected component (if applicable):
Satellite 6.3

How reproducible:
Satellite GUI > Content > Product > Repository > Sync Now

~~~
Type                        Docker
Registry URL                https://gcr.io
Upstream Repository Name    google_containers/kube-controller-manager-amd64 
~~~

Actual results:
Fails to sync
---
pulp_tasks:
- exception: 
  task_type: pulp.server.managers.repo.sync.sync
  _href: "/pulp/api/v2/tasks/01144bec-41ed-46aa-ab5a-1d034cb814c7/"
  task_id: 01144bec-41ed-46aa-ab5a-1d034cb814c7
  tags:
  - pulp:repository:19b81bf6-5fe8-4f53-8a6c-82f92d6efb93
  - pulp:action:sync
  finish_time: '2018-03-14T04:46:46Z'
  _ns: task_status
  start_time: '2018-03-14T04:46:35Z'
  traceback: |
    Traceback (most recent call last):
      File "/usr/lib/python2.7/site-packages/celery/app/trace.py", line 240, in trace_task
        R = retval = fun(*args, **kwargs)
      File "/usr/lib/python2.7/site-packages/pulp/server/async/tasks.py", line 527, in __call__
        return super(Task, self).__call__(*args, **kwargs)
      File "/usr/lib/python2.7/site-packages/pulp/server/async/tasks.py", line 107, in __call__
        return super(PulpTask, self).__call__(*args, **kwargs)
      File "/usr/lib/python2.7/site-packages/celery/app/trace.py", line 438, in __protected_call__
        return self.run(*args, **kwargs)
      File "/usr/lib/python2.7/site-packages/pulp/server/controllers/repository.py", line 769, in sync
        sync_report = sync_repo(transfer_repo, conduit, call_config)
      File "/usr/lib/python2.7/site-packages/pulp/server/async/tasks.py", line 730, in wrap_f
        return f(*args, **kwargs)
      File "/usr/lib/python2.7/site-packages/pulp_docker/plugins/importers/importer.py", line 83, in sync_repo
        self.sync_step = sync.SyncStep(repo=repo, conduit=sync_conduit, config=config)
      File "/usr/lib/python2.7/site-packages/pulp_docker/plugins/importers/sync.py", line 89, in __init__
        raise PulpCodedException(error_code=error_codes.DKR1008, registry=url)
    PulpCodedException: Could not find registry API at https://gcr.io

Expected results:
google container registry should be synced similar to docker or redhat public registry.

Additional info:
--Docker registry to sync--
gcr.io/google_containers/kube-apiserver-amd64 v1.5.1
gcr.io/google_containers/kube-controller-manager-amd64 v1.5.1
gcr.io/google_containers/kube-proxy-amd64 v1.5.1
gcr.io/google_containers/kube-scheduler-amd64 v1.5.1
gcr.io/google_containers/kubernetes-dashboard-amd64 v1.5.0
gcr.io/google_containers/etcd-amd64 3.0.14-kubeadm