iscsi role misses 'storage' group

Cinder-backup containers require iscsid to mount iscsi volumes
to backup if the volumes are on different storage host. This
fix adds missing 'storage' group to iscsi playbook.

Change-Id: Iba3fb861b3f14c20b8a020075f2473ba7a0dd9a2
Closes-Bug: 1611330

ansible/roles/iscsi/tasks/start.yml

@@ -18,7 +18,8 @@
       - "/sys/kernel/config:/configfs"
       - "cinder:/var/lib/cinder"
       - "iscsi_info:/etc/iscsi"
-  when: inventory_hostname in groups['compute']
+  when: inventory_hostname in groups['compute'] or
+        inventory_hostname in groups['storage']
 
 - name: Starting tgtd container
   kolla_docker: