Reduce DB calls when getting an image

Right now if you want to get a single image, two queries need to be
executed:
- Get image + associated locations and properties
- Get associated tags for image

Proposed solution combines these two queries into single one which gets
image and all related data - locations, properties and tags. Similar
pattern has been already implemented when querying for list of images.

Change-Id: Ic4424bb1eb1769d8f621ebe111d95961ecf08479
Closes-Bug: 1434578

glance/db/simple/api.py

@@ -380,7 +380,8 @@ def _sort_images(images, sort_key, sort_dir):
     return images
 
 
-def _image_get(context, image_id, force_show_deleted=False, status=None):
+def _image_get(context, image_id, force_show_deleted=False, status=None,
+               return_tag=False):
     try:
         image = DATA['images'][image_id]
     except KeyError:
@@ -396,12 +397,18 @@ def _image_get(context, image_id, force_show_deleted=False, status=None):
         LOG.warn(_LW('Unable to get unowned image'))
         raise exception.Forbidden("Image not visible to you")
 
+    if return_tag:
+        image['tags'] = image_tag_get_all(context, image_id)
+
     return image
 
 
 @log_call
-def image_get(context, image_id, session=None, force_show_deleted=False):
-    image = _image_get(context, image_id, force_show_deleted)
+def image_get(context, image_id, session=None, force_show_deleted=False,
+              return_tag=False):
+    image = _image_get(context, image_id, force_show_deleted,
+                       return_tag=return_tag)
+
     return _normalize_locations(context, copy.deepcopy(image),
                                 force_show_deleted=force_show_deleted)