hostname & subnode-hostname is taken from targets instead of labels

- If you see script nodepool/nodepool.py 1267 you will see that
  hostname & subnode-hostname properties read the attributes
  from the target section from the nodepool.yaml file instead
  of the labels section.

Change-Id: I74ef1e5ce01cc61e1e011c5fe5c7df7000fba2fe
Merge "hostname & subnode-hostname is taken from targets instead of labels"