Add option in check-load.rb to divide load averages by cpu/core count

Using `--per-core` option will divide system load averages by the number of cpus (cores/hyperthreads) the system believes it has. Allows the ability to specify uniform warning/critical thresholds across machines of varying cpu/core counts.

For example, a 1.0 load using this option assumes the system is at 100% capacity regardless of whether it is a single vcpu virtual machine or a multi-cpu, multi-core server with hyperthreading.