Add eventlogging::plugin custom resource type

This patch adds a custom resource type, 'eventlogging::plugin'. EventLogging
plug-ins are Python scripts which register themselves as handlers of some URI
scheme and which are placed in /usr/local/lib/eventlogging. If an EventLogging
service is directed at a stream that has a URI with a scheme matching the
plug-in's, EventLogging will delegate the handling of the stream to that
plug-in.

Change-Id: I3feae1458e99d30e22bdcc49ab04a5eadead1168