Set the nodepool user and key on dib images

We need to set the nodepool user and key on dib images so that dib can
run the pre use check script on the nodes (without these values nodepool
uses invalid defaults). Logically this appears to be part of the image
building process but the post slave boot check scripts need it too.

Change-Id: I51c7da1d58c2550903ad1671dcfb477b06424cfd
Merge "Set the nodepool user and key on dib images"