teach estatsd to support a custom name intead of just node()

estatsd gets the node name from node() directly. If running epmd and connecting the erlang node to the network isn't possible, this means the default atom nonode@nohost is used.

This patch enables estatsd to use a new application environment VmName, which if present, is used in NodeKey instead. This allows nice, and unique, metric names in graphite without needing epmd and connecting the node to the erlang network.