Description of problem:
cluster admin prune images without namespace limit can prune image, but it does not work when add flag "-n" or "--namespace"

Version-Release number of selected component (if applicable):
Both openshift and oadm: v3.4.0.39

How reproducible:
Always

Steps to Reproduce:
Use cluster-admin user login on cli locally:
master side: $ oadm policy add-cluster-role-to-user cluster-admin xiaocwan
local side: # oc login -u xiaocwan -p password

1. # oc new-project xiaocwan-st
2. # oc process -f https://raw.githubusercontent.com/openshift/origin/master/examples/sample-app/application-template-stibuild.json | oc create -f -

3. Delete all resources as below:
oc delete is/origin-ruby-sample
oc delete is/ruby-22-centos7
oc delete dc/database
oc delete dc/frontend
oc delete rc/database-1
oc delete rc/frontend-1
oc delete svc/database
oc delete svc/frontend

4. # oadm prune images 
Dry run enabled - no modifications will be made. Add --confirm to remove images

Deleting registry layer blobs ...
BLOB
sha256:0dd4f812e6cca8d571cca550c1b9e46c7db8b9ca3172a542bd3d6161eb2ae900

Deleting images from server ...
IMAGE
sha256:78fc445678f3d22946a2ee79b12dbfe6f41def30cf61b6adee55c54036c937e4

5. # oc get image|grep 678f3d2294
sha256:78fc445678f3d22946a2ee79b12dbfe6f41def30cf61b6adee55c54036c937e4   172.30.206.2:5000/xiaocwan-st/origin-ruby-sample@sha256:78fc445678f3d22946a2ee79b12dbfe6f41def30cf61b6adee55c54036c937e4


6. # oadm prune images --namespace=xiaocwan-st (or -n xiaocwan-st)
Dry run enabled - no modifications will be made. Add --confirm to remove images

Actual results:
6. Prune images for namespace "xiaocwan-st" returns no image.

Expected results:
6. Prune images for namespace "xiaocwan-st" with dry-run mode should output same result as step 4

Additional info:
`oadm options` contains the flag for namespace: 
-n, --namespace=''