[autotest] Fixing and re-enabling monitor_db_functional_test.

The test was disabled and outdated. Database access and mocking of the drone
manager changed. This fixes these issues, updates the unit tests to the
current status and reanables them.

BUG=chromium:395756
DEPLOY=scheduler
TEST=ran ./utils/unittest_suite.py

Change-Id: I6a3eda5ddfaf07f06d6b403692b004b22939ffb6
Reviewed-on: https://chromium-review.googlesource.com/209567
Reviewed-by: Alex Miller <milleral@chromium.org>
Tested-by: Jakob Jülich <jakobjuelich@google.com>
Commit-Queue: Jakob Jülich <jakobjuelich@google.com>
diff --git a/scheduler/monitor_db_unittest.py b/scheduler/monitor_db_unittest.py
index 184619e..2ead35b 100755
--- a/scheduler/monitor_db_unittest.py
+++ b/scheduler/monitor_db_unittest.py
@@ -536,7 +536,7 @@
         self.god = mock.mock_god()
         self.mock_drone_manager = self.god.create_mock_class(
             drone_manager.DroneManager, 'drone_manager')
-        self.god.stub_with(pidfile_monitor, '_drone_manager',
+        self.god.stub_with(drone_manager, '_the_instance',
                            self.mock_drone_manager)
         self.god.stub_function(email_manager.manager, 'enqueue_notify_email')
         self.god.stub_with(pidfile_monitor, '_get_pidfile_timeout_secs',