Adds non_inheritable_image_properties flag.

Some image-properties should not be inherited from the instance when
taking a snapshot because we want that functionality to only available
for the base-image, not subsequent snapshots.

As an example, we would like 'cache_in_nova' fast-cloning to be
available just for base-images and not customer snapshots created off a
base-image.

This patch adds a new configuration which allows operators to, at
runtime, select which image-properties should not be inherited by newly
created snapshots.

Change-Id: I8d7781b05ffd71a59ba69fb7c3df4616cba94cce

nova/flags.py

@@ -414,6 +414,10 @@ global_opts = [
     cfg.StrOpt('auth_strategy',
                default='noauth',
                help='The strategy to use for auth: noauth or keystone.'),
+    cfg.ListOpt('non_inheritable_image_properties',
+                default=['cache_in_nova'],
+                help='These are image properties which a snapshot should not'
+                     ' inherit from an instance'),
 ]
 
 FLAGS.register_opts(global_opts)