Arista L3 Ops is success if it is successful on one peer

This fix checks to see if Arista HW is
configured in MLAG (redundant) mode. If yes,
as long as operation is successful on one of the
paired switches, consider it successful.

Closes-bug: 1373652

Change-Id: If929d3fcc109b81f4cf071380a1645d403feb363
Merge "Arista L3 Ops is success if it is successful on one peer"