When the configdrive input is JSON (meta_data, etc), delay the rendering
until the ISO image is actually used. It has two benefits:
1) Avoid storing a large ISO image in instance_info,
2) Allow deploy steps to access the original user's input.
Fix configdrive masking to correctly mask dicts.
Story: #2008875
Task: #42419
Change-Id: I86d30bbb505b8c794bfa6412606f4516f8885aa9