Fix default value for interface plugin

The interface plugin default was defaulting to a string type and the rendered
output was converting that to a list of single characters. Switch the default
to a list with a single entry.
Add support for collectd ping plugin

The ping plugin allows you to monitor the ping latency of a list of hosts that
you specify. I've added the state support and example pillar configuration.
Merge pull request #1 from iggy/f_ping_plugin

F ping plugin