Enhance the collector_plugins option help text
Enhance the collector_plugins option help text

This enhances the [collector]/collector_plugins
config option help text to mention the storage
and baremetal in-tree collectors and the ability
to load out-of-tree collectors via extension point.

While doing this, the help text is formatted for
prettier rst rendering in the docs.

Change-Id: Ifd32c95c664c4e9586c250e6bceaeaba2e2df417

openstack/watcher,stackforge/watcher,openstack/watcher,stackforge/watcher