Fix the setup-loopback-device service config for upstart

When we use the upstart, setup-loopback should be running as a
service to pass the salt linux.storage.loopback state
(service.running).

Change-Id: I9b19a94126b497d89bd6d05f43dca1df0304c0c6
Closes-bug: PROD-19847

linux/files/setup-loopback-device.upstart

 description "Setup {{ device_name }} device"
 
 start on filesystem
-task
 
-pre-start script
-  if /sbin/losetup {{ device_name }}; then
-    stop ; exit 0
-  fi
-end script
+pre-start exec losetup {{ device_name }} {{ file }}
+post-stop exec losetup -d {{ device_name }}
 
-exec losetup {{ device_name }} {{ file }}
+script
+  while losetup {{ device_name }} ; do sleep 60 ; done
+end script