Make Graphite and Ganglia optional dependencies

This ticket started out as "Provide a way to not specifically depend on and package the various reporters tied to the Gremlin Server `MetricsManager`", but while that is a nice idea, it ends up being more complicated than what is required and at this point I haven't really heard of any users asking for the ability to drop in other reporters.

Anyway, Ganglia/Graphite should be made optional dependencies as they aren't really required for operation of Gremlin Server and it should be up to the user to decide which they want to use and include. Plus, this gets rid of the ugly problem of an LGPL dependency which we used to just exclude and thus force the user to copy in manually. As a result, we kinda added stuff to the distribution which didn't work out of the box anyway. Better to just avoid the whole issue and not include them.