The expectations of MAASDispatcher.dispatch_query are not tested

[Gavin Panella]

The return value from dispatch_query is defined as:

  :return: A open file-like object that contains the response.

However, code that uses it depends on the fact that it's the return
value of urllib2.urlopen, which is a urllib.addinfourl instance, with
methods like getcode, info, and geturl.

The real problem is that none of this is tested, so a recent change to
use httplib2 instead of urllib2 passed the test suite with minimal
alterations, but broke start_cluster_controller.