Add Node.maintenance_reason

When a node is maintenanced by Ironic or an operator, an operator
should know the reason for that. Add a string column to allow
Ironic or operators to enter a reason that the node is in
maintenance mode.

Implements: blueprint maintenance-reason

Change-Id: I7c77c92501e53f81704e9b9e5b52e0e2a410cc91
Merge "Add Node.maintenance_reason"