Work around iscsiadm delete failures

Attempting to delete an iSCSI target immediately after the initiator has
logged out of the session can sometimes cause iscsiadm's database update
to fail.  This allows the delete command to be retried up to 5 times on
failure.  We also accept exit code 21 (no records found) as success.

Closes-bug: #1324670
Change-Id: I064fcd01e3bd713b0ec19cc1fdb562ca7fd8b666
Merge "Work around iscsiadm delete failures"