connector auto-discovery and initialization/shutdown of connectors

This change allows the cloud connector classes to be automatically discovered via the ServiceLoader features of Java 1.6+.  The auto-discovery uses a "stub" class for each cloud connector type to find and load the relevant classes.  The "stub" classes also have methods for the initialization/shutdown for the cloud connector classes.  These are invoked once only at SlipStream service startup and shutdown.  This is used, for example, to start/stop the job queue for the credential renewal for the OCCI connector.