Create a backend_random director

There are many requests for CentralAutoLogin/start, which are
(currently) not cached, but all go to the same backend Varnish
instance due to the URL getting hashed by the chash 'backend'
director. Create another 'backend_random' director with the same
backends for these requests.

Change-Id: Iea09d8e8da703ec330ecb129d3ed5ec690cb4949
Merge "Create a backend_random director" into production