JOC Cockpit should allow to disable Agent Clusters

Current Situation

• Users would want to disable Agent Clusters, e.g. in case of maintenance windows.
  • Disabling an Agent Cluster prevents any Agents in that cluster from being used.
  • Any task start requests towards that Agent Cluster should be enqueued and should be delayed to a later point in time when the Agent Cluster once again is enabled.
• The behavior is different from automated Agent detection as performed by the Master due to the fact that the user decides at what point in time an Agent Cluster can be used. This is intended to be independent from the availability of an Agent Cluster.

Desired Behavior

• The JOC Cockpit should offer two operations with the "Resources->Agent Clusters" view to disable and enable an Agent Cluster.
• Technically the max_processes attribute of the Agent Cluster should be set to 0 to disable an Agent Cluster.
• Enabling a disabled Agent Cluster includes to reset the value of the max_processes attribute to its original value.
• Changes to an Agent Cluster are stored persistently with the *.process_class.xml configuration file of the Agent Cluster.
• Disabling/enabling of Agent Clusters should be based on a single new permission.