Feature/cluster mode unique server for job schedules

Add very simple support for multi-server rundeck configuration pointing at the same DB backend, allowing scheduled (quartz) jobs to execute only on a single node in the "cluster".

* Requires the feature to be explicitly enabled with `rundeck.clusterMode.enabled=true` in rundeck-config file
* Requires a `rundeck.server.uuid` to be set for each node in the framework.properties

With this enabled, Scheduled (quartz) jobs will not attempt to run on every server node, but will only be scheduled on the server they were last modified on. In the case where a job is scheduled on server A, but modified and saved on server B, the quartz job when run will detect the changed serverNodeUUID for the job, and unscheduled itself on node A, while server B will run the job correctly.

This supports a basic failover mode, using multiple Rundeck servers pointing at a single DB.