cluster_run: Don't change OS X NOFILE rlimit if it's already >2048

Change-Id: Ia099498991b5c7673a65667b66aa67b7c614c0e5
Reviewed-on: http://review.couchbase.org/38550
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>