Trap iscsiadm error

If the iscsitarget was previously deleted and the user tries
to delete the iscsi target again, then you get a nasty traceback
in the logfiles.

According to the iscsiadm man page, iscsiadm returns an error 21,
ISCSI_ERR_NO_OBJS_FOUND if there is no records/targets/sessions/portals
found to execute the operation on.

LP: #1011166

Change-Id: I3087c1a114fe91bd7361d92688d4f926bc3f8951
Signed-off-by: Chuck Short <chuck.short@canonical.com>

nova/virt/libvirt/volume.py

@@ -203,4 +203,4 @@ def disconnect_volume(self, connection_info, mount_device):
             self._run_iscsiadm(iscsi_properties, ("--logout",),
                                check_exit_code=[0, 255])
             self._run_iscsiadm(iscsi_properties, ('--op', 'delete'),
-                               check_exit_code=[0, 255])
+                               check_exit_code=[0, 21, 255])