Graphite plugin should support distinct bind interface

[otoolep]

This probably applies to other plugins too, but I already have a need for this. I want to expose the Admin interface over 0.0.0.0, but I do not want Graphite to bind to 0.0.0.0 -- I want it to only be available to collectd running locally. However, the Graphite plugin uses the same bind address as everything else. This is security issue for me, with my cloud-based instances.

I can work around this by putting nginx in front of influxdb, and having influxdb listen to localhost, but I could see other uses where the plugin bind interfaces are different than the admin interface (a machine with two NICs, for example).

[otoolep]

I suggest we support a new config option in the graphite plugin section, that allows the bind address to be set. If not set, then use the bind-address config set at the top-level. This means the default behaviour will be the same as we currently have.