allow setting of graphite options

gmetad now support pushing stuff data directly to carbon aggregator.
I added the setting to gmetad so you can set them in the gmetad.conf

also this one sneaks in a change so you can set `write_rrds` as well (by default it should behave the same way as master).  For some reason someone may want to disable rrds.
Would it make sense to only write these lines if the attributes are set? i.e. do something similar the two-gmetad setup where there's an enabling attribute (node['gangila']['gmetad']['enable_graphite'] or something) and then wrap these three lines in a conditional dependent on that variable. If not, I'd love to know what happens if these are set to empty strings (and the default should probably be off, same as the two gmetad config)