add the ability to remove nodes older than a certain age even when the node group has reached the min

[awprice]

• adds max_node_age field to node group configuration
• max_node_age by default is disabled, and needs to be a positive, greater than zero duration to enable
• when the node group has reached the minimum and there are nodes older than the max_node_age, the scale delta will be set to +1
• this will then force escalator to replace the oldest node

fixes #239