Disable caching in Parsoid front-ends

To avoid missing request coalescing in the backends when forcing misses with
req.hash_always_miss = true, we now plan to send updates only to one of the
front-end caches (via the load balancer). For this to work the front-ends
should not cache themselves. At the low request rates (< 100 req/s, ~0.5%
CPU..) we don't need those caches anyway.

Tested in the labs install.

Change-Id: I662070da66d091586700bb0b0a0f89354f0926cc
Merge "Disable caching in Parsoid front-ends" into production