Setting a member as "backup" means no traffic will be sent to it unless
all non-backup members are marked as down.
This should be essentially the same in every backend provider AFAIU.
This was requested by at least one operator (me) and was agreed during
the PTG to add value.
Story: 2001777
Task: 12483
Change-Id: I953abe71a0988da78efc6b3961f7518c81c2a06d