Add short buffer between backup delete

For backup delete under retention.
It's possible to have delete command conflict with
wrong process/accept order in Cinder.

This patch add 2 seconds sleep time to give Cinder a short break to
accept API calls in correct order.
diff --git a/staffeln/conductor/manager.py b/staffeln/conductor/manager.py
index 33ab139..62530c0 100755
--- a/staffeln/conductor/manager.py
+++ b/staffeln/conductor/manager.py
@@ -267,6 +267,7 @@
                                 self.controller.hard_remove_volume_backup(

                                     backup, skip_inc_err=True

                                 )

+                                time.sleep(2)

             except coordination.LockAcquireFailed:

                 LOG.debug("Failed to lock for retention")