Graphite/Carbon: set max open files via Upstart 'limit' directive

Upstart does not respect /etc/security/limits.{conf,d}. If you want to set
limits for Upstart jobs, you have to do it explicitly in the job configuration,
via the 'limit' directive.

This patch set ulimit -n to 65k for Carbon instances and removes the limits.d
file. I'll purge the file from tungsten manually.

Change-Id: I4fa1ccf58ff700d4521a5cf19de5309b57aa9296