Load plug-in data sinks from preconfigured location

This patch allows EventLogging to be extended using plug-ins. Plug-ins are
Python scripts that register data readers/writers using the @reads and @writes
decorators provided in eventlogging.factory. If these Python scripts are placed
in /usr/lib/eventlogging (overrideable by setting EVENTLOGGING_PLUGIN_DIR in
the environment), they will be auto-loaded by eventlogging-consumer, unless
'--no-plugins' is specified on the command line.

Change-Id: Ia5f9d9979e740ef903be2611fb482e1849aec16d