Fix l3 lag test failure

Fix l3 lag test failure

Ticket: CM-1438
Reviewed By:
Testing Done: l3 lag test with help from purna

- THe down sequence in the new ifupdown was causing switchd some grief
  (wilson is looking at it). readded the topological sort which i had
removed in favor of only walking the tree. With the fix,i dont see the
switchd problem anymore.
- And another down bug was causing the bond to go away prematurely (only
  with the all depends option). Added a upperdevice list to track upperdev references