Replace global Conf mangling with monkeypatch

The test suite, especially for the brokers, was heavily dependant on the
success of the tests preceeding it to pass. This commit should eliminate
that dependency and allow tests to fail without affecting others.
It also removes the need to cleanup globals manually after a test.

django_q/tests/test_monitor.py

@@ -9,7 +9,7 @@ from django_q.conf import Conf
 
 
 @pytest.mark.django_db
-def test_monitor():
+def test_monitor(monkeypatch):
     assert Stat.get(0).sentinel == 0
     c = Cluster()
     c.start()
@@ -25,14 +25,13 @@ def test_monitor():
             break
     assert found_c is True
     # test lock size
-    Conf.ORM = 'default'
+    monkeypatch.setattr(Conf, 'ORM', 'default')
     b = get_broker('monitor_test')
     b.enqueue('test')
     b.dequeue()
     assert b.lock_size() == 1
     monitor(run_once=True, broker=b)
     b.delete_queue()
-    Conf.ORM = None
 
 
 @pytest.mark.django_db