Add plugins recipe

The intent of this recipe is to allow users to install plugins either by
putting them in their copy of the cloudmonitoring cookbook or by
creating a new cookbook containing their plugins. The style is
essentially copied from the ohai cookbook.
Document plugins recipe
Make resource name unique

Fixes warning about cloning a resource CHEF-3694

```
WARN: Cloning resource attributes for remote_directory[/usr/lib/rackspace-monitoring-agent/plugins] from prior resource (CHEF-3694)
```
Explain why the plugins hash is slightly weirdly assigned