ganglia wrapper for py plugins

There are several python plugins in our repository and we end up
writing the same puppet glue every time: install the python script and
its default configuration file, making sure gmond is notified.

This patch introduces a ganglia::python::plugin define taking the plugin name
as title. One can optionally pass a hash 'opts' that can be reused in the
template. See define documentation.

Change-Id: If2f001ffa296dc7b67f53c56d9fbcf96cb5dba2a