Adding a configuration flag:  enable_pnp4nagios

I have added a new config flag: enable_pnp4nagios.  This behaves similarly to enable_graphite, and removes unwanted elements from the status views if pnp4nagios is not in use.

I have never used github before today, so please be gentle if I have broken any etiquette, here.

Thanks,
Pete Morgan aka smartattack on #adagios