Adding two carbon-cache workers to graphite.

Graphite has had its share of problems in the last couple of days;
since it seems to originate from a slowness in getting data from
carbon-cache, and the workers are overloaded (100% CPU on tungsten), we
think this may ease the load and allow to make graphite-web and gdash
usable again.

Change-Id: I57d5d85bfaad6133aab5fac3e0638b95cc2bf526
Signed-off-by: Giuseppe Lavagetto <glavagetto@wikimedia.org>