proxyquery: factor source header processing

This block is now in a separate function, in order to reduce the overall
size and complexity of `post_job`.
proxyquery: rename cluster config loading funcs
proxyquery: simplify and refactor post_job
Move a ClusterConfigParser method to ZvmNode

`get_list_of_remote_objects` operates only on a ZvmNode, and it seems
more appropriate to just make it an instance method of ZvmNode.
proxyquery: encapsulate cluster config parsing
proxyquery: remove a redundant variable