Updating ocp es proxy image to use openshift_logging_proxy_image_prefix if specified

roles/openshift_logging_elasticsearch/vars/openshift-enterprise.yml

@@ -1,3 +1,3 @@
 ---
-__openshift_logging_elasticsearch_proxy_image_prefix: "registry.access.redhat.com/openshift3/"
+__openshift_logging_elasticsearch_proxy_image_prefix: "{{ openshift_logging_image_prefix | default('registry.access.redhat.com/openshift3/') }}"
 __openshift_logging_elasticsearch_proxy_image_version: "v3.7"