Do not fail when action is delete and volume is not present

This prevents failing when a playbook describes a volume deletion and
is launched more that once.

Without this fix, if you run the playbook a second time, it will fail.

system/gluster_volume.py

@@ -433,7 +433,7 @@ def main():
         else:
             module.fail_json(msg='failed to create volume %s' % volume_name)
 
-    if volume_name not in volumes:
+    if action != 'delete' and volume_name not in volumes:
         module.fail_json(msg='volume not found %s' % volume_name)
 
     if action == 'started':