Refactor installation recipes.

It should now be possible to customise the install process without
making local changes to this cookbook.

Recipes which are intended to be included from other recipes are named
with a leading underscore.
Don't notify server from common recipe.

We might only be installing the client, which causes the notify to
fail.  This introduces a bug, in that the service should be notified
of common changes if it is running on the node.
Create site-plugins directories in common recipe.
Expose location of plugin.d as an attribute.
Update metadata to pass Foodcritic FC045.
Update common recipe to pass Foodcritic FC002.
Correctly support RHEL-family distros per Foodcritic FC024.
Additional configuration parameterisation.

Additions:
 * direct_addressing
 * YAML fact file location
 * classesfile location
 * Identity
Parameterise collective membership.
(FC012) Convert README to markdown format.
Minor updates to metadata.
Fix handler to respect factfile and classfile attributes.
Tidy up CHANGELOG ahead of release.