Add methods to get image metadata from instance

This patch adds a couple of utility functions that enclose all the logic
for getting and parsing the image metadata stored in the instance's
system metadata.

First, this will try to fetch the metadata from the real image and will
prevent it from failing if it is not available. It will be then merged
with the image metadata stored during the instance creation.

Related to bug #1039662

Change-Id: I2130caf19858585571b1199e27f0a98ad5f08701

nova/compute/utils.py

@@ -225,6 +225,29 @@ def _get_unused_letter(used_letters):
     return letters[0]
 
 
+def get_image_metadata(context, image_service, image_id, instance):
+    # If the base image is still available, get its metadata
+    try:
+        image = image_service.show(context, image_id)
+    except Exception as e:
+        LOG.warning(_("Can't access image %(image_id)s: %(error)s"),
+                    {"image_id": image_id, "error": e}, instance=instance)
+        image_system_meta = {}
+    else:
+        instance_type = flavors.extract_flavor(instance)
+        image_system_meta = utils.get_system_metadata_from_image(
+                image, instance_type)
+
+    # Get the system metadata from the instance
+    system_meta = utils.instance_sys_meta(instance)
+
+    # Merge the metadata from the instance with the image's, if any
+    system_meta.update(image_system_meta)
+
+    # Convert the system metadata to image metadata
+    return utils.get_image_from_system_metadata(system_meta)
+
+
 def notify_usage_exists(notifier, context, instance_ref, current_period=False,
                         ignore_missing_network_data=True,
                         system_metadata=None, extra_usage_info=None):