Make a few places tolerant of sys_meta being a dict

This will be necessary as we start to introduce objects with
real dicts in system_metadata. These are the common spots that
need to be ready ahead of time. The rest are at the actual use
sites and can/will be replaced when individual uses are
converted to objects.

Related to bp/unified-internal-objects

Change-Id: I8314e0d52ec2ae800765f60ce58ce9b309d7d513

nova/compute/flavors.py

@@ -238,7 +238,7 @@ def extract_instance_type(instance, prefix=''):
     information."""
 
     instance_type = {}
-    sys_meta = utils.metadata_to_dict(instance['system_metadata'])
+    sys_meta = utils.instance_sys_meta(instance)
     for key, type_fn in system_metadata_instance_type_props.items():
         type_key = '%sinstance_type_%s' % (prefix, key)
         instance_type[key] = type_fn(sys_meta[type_key])

nova/notifications.py

@@ -288,8 +288,7 @@ def null_safe_str(s):
     instance_type_name = instance_type.get('name', '')
 
     if system_metadata is None:
-        system_metadata = utils.metadata_to_dict(
-                instance_ref['system_metadata'])
+        system_metadata = utils.instance_sys_meta(instance_ref)
 
     instance_info = dict(
         # Owner properties

nova/utils.py

@@ -1032,6 +1032,13 @@ def dict_to_metadata(metadata):
     return result
 
 
+def instance_sys_meta(instance):
+    if isinstance(instance['system_metadata'], dict):
+        return instance['system_metadata']
+    else:
+        return metadata_to_dict(instance['system_metadata'])
+
+
 def get_wrapped_function(function):
     """Get the method at the bottom of a stack of decorators."""
     if not hasattr(function, 'func_closure') or not function.func_closure: