Define node["collectd"]["plugins_conf_dir"] attribute

Hi,

When I'm using your collectd cookbook with distribution packages (Ubuntu 14.04 LTS) I can't use ```collectd_plugin```LWRP because it assumes collectd is going to be installed at ```/opt/collectd``` and its conf directory will be inside that directory.

I'm proposing to have a new attribute to indicate where are going to be located plugins configuration files.

Thanks,