#134 Multiple Graphite instances: support for items 1 and 2

This set of commits replaces the Graphite fields in SeyrenConfig with a list of GraphiteInstanceConfig configuration objects. Right now SeyrenConfig still creates just a single GraphiteInstanceConfig (based on the existing GRAPHITE_XXX environment variables), so no change there. But now the rest of the app sources Graphite configuration from the GraphiteInstanceConfigs.

Need to establish the desired approach for configuring the multiple Graphite instances. I would suggest JSON or YAML as we discuss in issue #134. Once we have that we can move forward with updating the UI to take advantage of the underlying multi-Graphite support.