Glance: Fix clean-secrets script to ignore not found error

Fixed clean-secrets script to avoid deleting glance service failure
when rbd pool secret does not exist.

Change-Id: I9c47d03d0775332c67198ce79b4ca7b7a491a9a4

glance/templates/bin/_clean-secrets.sh.tpl

@@ -18,4 +18,7 @@ limitations under the License.
 
 set -ex
 
-exec kubectl delete secret --namespace ${NAMESPACE} ${RBD_POOL_SECRET}
+exec kubectl delete secret \
+  --namespace ${NAMESPACE} \
+  --ignore-not-found=true \
+  ${RBD_POOL_SECRET}