Add node-set-maintenance command

This command accepts an optional reason and calls the new
/nodes/uuid/maintenance endpoint to set and clear maintenance
mode and maintenance reason.

It will reject setting maintenance mode to off with a reason,
as setting maintenance mode to off clears the reason.

Change-Id: I04e14bc239b343d8791747963e4feec1e2cbdda8
Implements: blueprint maintenance-reason
Merge "Add node-set-maintenance command"