Task cleanup_running_deleted_instances can now use slave

Deployments that have a slave_connection configured can offload
read queries generated by this periodic task there. For
deployments with no slave, behavior remains the same.

Partially-implements: blueprint periodic-tasks-to-db-slave
Change-Id: Ie1e6cdfbefab096a2af62ef636786a5c8f659b13
Merge "Task cleanup_running_deleted_instances can now use slave"