add a "max age" argument for node candidacy

[jackfrancis]

When determining which node to pick as a prototype candidate, if we determine that there are no new OS updates on any node, we can fall back on a "max age" criterion to potentially pick a node. It will go like this:

• If no nodes have any OS updates compared to the current VMSS OS image
  1. Compare the age of that current VMSS OS image to the "max age" configuration
  2. Is the "max age" less than the age of the current VMSS OS image?
  3. If so, pick a node using the other existing node candidacy criteria.