Move ZvmNode from common to configparser
Clean up `json` module imports
ClusterConfigParser: clean up node collections

Both a list and a dict of the nodes were being maintained. This is
unnecessary.
ZvmNode: refactor _create_node

Instead of the standalone util function, _create_node is now a
classmethod of ZvmNode called `fromdict`.
proxyquery: remove commented-out code
Merge pull request #166 from larsbutler/cleanup-ClusterConfigParser

Clean up ClusterConfigParser