convert virt drivers to fully dynamic loading

Progress on bp:virt-driver-cleanup

This series of patches converts the virt drivers to loading via
importutils making it possible to add a virt driver without changing
core code. It deprecates the use of connection_type in favor of a
full driver name in compute_driver.

Based on email thread with Vish and Jay Pipes on approaches to do
this.

Change-Id: I48366ec7efc7b095859988e5e6ac5a853b98b8a7

nova/test.py

@@ -152,10 +152,6 @@ class TestCase(unittest.TestCase):
             self.mox.VerifyAll()
             super(TestCase, self).tearDown()
         finally:
-            if FLAGS.connection_type == 'fake':
-                if hasattr(fake.FakeConnection, '_instance'):
-                    del fake.FakeConnection._instance
-
             if FLAGS.image_service == 'nova.image.fake.FakeImageService':
                 nova.image.fake.FakeImageService_reset()