Increase count of carbon cache instances to eight

tungsten has 16 CPUs with four cores each. Apart from the six carbon-cache
instances, the jobs that generate significant load are mwprof (one process,
single-threaded), txStatsD (one process, single-threaded), graphite-web
(four uWSGI worker processes), and carbon-relay (one process, single-threaded).
Profiling the system shows that we are not exploiting parallelism as much as we
could -- whenever I looked there are always multiple processors that are just
idling. If the incoming Graphite metric load was enough to overwhelm four
instances, let's not stop six.

Change-Id: I978efd095392c0e262a22ad7deca6cff9d1bb736
Merge "Increase count of carbon cache instances to eight" into production