Correct use of label.providers dict

* nodepool/nodepool.py: Fix attempts to access nonexistent "name"
attribute of keys in the label.providers dict. Also normalize other
uses of this dict for consistency to access provider names as the
"name" attributes of the provider objects which are its values.

Change-Id: If57d73fa994257dbf069c53289585a9c0a375eb4
Merge "Correct use of label.providers dict"