Fix up some feedback on image precache support

This addressed a few feedback items from earlier in the stack.

Related to blueprint image-precache-support

Change-Id: I622a9180d7b53dd35e60e2335fe185da1d6ac019

nova/conductor/manager.py

@@ -1660,7 +1660,7 @@ class ComputeTaskManager(base.Base):
             hosts_by_cell[hmap.cell_mapping.uuid].append(hostname)
 
         LOG.info('Preparing to request pre-caching of image(s) %(image_ids)s '
-                 'on %(hosts)i hosts across %(cells)s cells.',
+                 'on %(hosts)i hosts across %(cells)i cells.',
                  {'image_ids': ','.join(image_ids),
                   'hosts': len(aggregate.hosts),
                   'cells': len(hosts_by_cell)})
@@ -1688,6 +1688,8 @@ class ComputeTaskManager(base.Base):
         fetch_pool.waitall()
 
         clock.stop()
+        # FIXME(danms): Calculate some interesting statistics about the image
+        # download process to log for the admin.
         LOG.info('Image pre-cache operation for image(s) %(image_ids)s '
                  'completed in %(time).2f seconds',
                  {'image_ids': ','.join(image_ids),

nova/tests/unit/virt/libvirt/test_driver.py

@@ -23561,7 +23561,10 @@ class LibvirtDriverTestCase(test.NoDBTestCase, TraitsComparisonMixin):
         else:
             mock_et.assert_not_called()
 
-    def test_cache_image_existing_first_time(self):
+    def test_cache_image_uncached_first_time(self):
+        # Test the case where we do need to download the image,
+        # and this is the first time an image cache operation has ever
+        # been performed, so the directory structure has to be created.
         self.test_cache_image_uncached(first_time=True)
 
     @mock.patch('oslo_utils.fileutils.ensure_tree')