Allow prefix configuration for graphite

When running in a graphite configuration, it could be interesting to be able to use a prefix for key name, to separate different application in the same graphite database.
I prefer to create séparate key by application by default, to isolate generic values ( jvm memory,  threads, .. )

For testing purpose, I use a docker image to test :  https://github.com/hopsoft/docker-graphite-statsd
Works very well :)