Add FlushNodes, UpdateConfiguration, and KillManager actions to the manager

[CGodiksen]

Currently nodes can be flushed individually and settings in the configuration can be updated on nodes individually. Actions should be added to the manager that makes it possible to flush all nodes in the cluster, and update a setting for all nodes in the cluster. Furthermore, an action should be added to kill the manager (similarly to KillNode), which removes every node in the cluster (killing every node) and then stops the process of the manager.

• Add action on manager to flush all nodes in the cluster.
• Add action on manager to update configuration for all nodes in the cluster.
• Add action on manager to kill all nodes in the cluster and kill the manager process after.