high: ui_maintenance: Add maintenance [reload] sublevel (bnc#899234)

This adds a new sublevel: maintenance

Commands in this sublevel are only valid when the cluster is in
maintenance mode, or the affected resource is in maintenance mode.

This patch also adds two commands in this sublevel:

 - action: invoke a resource action directly using the agent
 - actionssh: invoke a resource action on multiple nodes using pssh