settings.GRAPHITE_ON -> settings.enable_graphite

This is more in sync with the naming convention inside settings.py
more specifically:

* stuff that is supposed to be configurable, is lowecase
* On/Off switches are prefixed with enable_*
* Made it off by default, since minority of users have graphite installed

I proposed a naming convention here:

https://github.com/opinkerfi/adagios/wiki/Developers-Guide#configurables-and-settingspy
settings.GRAPHITE_URL -> settings.graphite_url
settings.GRAPHITE_PREFIX -> graphite_prefix
Add graphite settings to adagios.conf
Add graphite options to /misc/settings