Tuneable, optional storage-schemas/aggregation templates

Hi,
See https://github.com/hw-cookbooks/graphite/issues/40

* Default behavior is unchanged.
* Set ['graphite']['storage_schemas'] or ['storage_aggregation'] to *nil* to skip handling, but empty array as value would generate an empty file.
* Array is used because order matters, ruby 1.8 hash iteration is not ordered (for non-omnibus chef with old ruby) & priority field is deprecated in graphite.
* Chef 11: setting non-nil attribute to nil actually sets the value to empty hash, which should be ignored also.


Oh... forgot to update docs - so if it's ok to merge, tell me and I'd add a doc update commit here.