Add a nodepool dib script to create /etc/nodepool

/etc/nodepool is required by nodepool to exist after nodes are booted.
We must then create it in our images. Add a finalize step to create this
dir as it isn't necessary until the node boots (not necessary for image
creation).

Change-Id: I42cc32a261a632d127963eb06333d1ef58f6d88d
Merge "Add a nodepool dib script to create /etc/nodepool"