Add initial files for certificate event handling

The SSL certificate generation and workflow blueprint called for adding
support for a certificate events plugin. This CR defines an interface for this
plugin, a simple default logging-based implementation, and an example
integration with the task resource logic.

Change-Id: Ia46793a1ce767ff03d77da06bf5a2803e7dce121
Implements: blueprint add-ssl-ca-support
Merge "Add initial files for certificate event handling"