Returning a tuple instead of list in list_extensions

extension_supported is decorated with
memoized_with_request(list_extensions, 1).
This means that list_extensions is called to build the args for
extension_supported and then make a hash.
list_extensions was returning a list which is unhashable.
We now return a tuple which has the following advantages:
1: is hashable.
2: tuples use less memory than lists in python, and should be use when we do
not need a mutable.

Change-Id: I92be6ef7880c6ac76847ec9cc8b2c83a6fd009b2
Closes-Bug: #1596545

openstack_dashboard/api/nova.py

@@ -1042,11 +1042,11 @@ def list_extensions(nova_api):
     """List all nova extensions, except the ones in the blacklist."""
     blacklist = set(getattr(settings,
                             'OPENSTACK_NOVA_EXTENSIONS_BLACKLIST', []))
-    return [
+    return tuple(
         extension for extension in
         nova_list_extensions.ListExtManager(nova_api).show_all()
         if extension.name not in blacklist
-    ]
+    )
 
 
 @memoized_with_request(list_extensions, 1)