write_graphite plugin: Change <Carbon> blocks to <Node> blocks.

write_graphite plugin: Change <Carbon> blocks to <Node> blocks.

Many other write plugins, e.g. write_{mongodb,redis,riemann} use the
<Node "name"> syntax. This adds code to handle this syntax as well.

While the documentation doesn't mention it, the old <Carbon> syntax
(without a name) is still supported for backwards compatibility.