add cmdline support for log_dir
logster MetricObjects can now take some Ganglia specific configs, like slope.

JsonLogster now implements a get_metric_object method that can be overridden
if you have custom attributes you need to set on the MetricObject for
a particular metric.

See this for an example usage:
https://gist.github.com/ottomata/7831961

Change-Id: I983000d95edcc9bc6e45b3bf11d650828169bf1c