Provide an override level for os-apply-config

This allows elements to override the templates provided by other
elements using templates in post-os-apply-config. For example, the
logstash element provides a very basic logstash.conf. This change allows
another element to easily substitute a more useful version.

README.md has been rewritten as it was fairly sparse to begin with.

Change-Id: I14a103bf16554dc42b9c8ba1d4fefcacf6f07786
Merge "Provide an override level for os-apply-config"