Add RPC method for node maintenance mode

Method 'change_node_maintenance_mode' added to manager and rpcapi.
This method triggered maintenance mode for a node.
New column 'maintenance' added to nodes table.

Partial-Bug: #1260099
Change-Id: I945a1ce72c04e5ee2a9427a58dae72b0719c160f
Merge "Add RPC method for node maintenance mode"