Fix use of bare list/dict types in instance_group object

The instance_group object was merged before we had helpers for
dict and list objects of strings. This makes that object use those
for schema safety.

Change-Id: I1f68e43a83f140f7b29b17c0961b267e64e48390
Closes-bug: 1221383
Merge "Fix use of bare list/dict types in instance_group object"