Avoid accessing private parsing method

[Duncan McGreggor]

In a review comment, Jamu made the following observation:

+ def test_parse_reservation(self):
+ ec2 = client.EC2Client(creds='foo')
+ results = ec2._parse_reservation(sample_describe_instances_result)
+ self.check_parsed_reservations(results)

It would be nice to avoid accessing the private _parse_reservation
method by passing a query_factory to EC2Client that returns a
FakeQuery. FakeQuery.submit could return a succeeded deferred and
you could call ec2.describe_instances to get a Deferred with the
result you want to make assertions about.