This provides an option to specify a swift container for stack
actions and all child templates and env files will be fetched
from the container, if available. However, files coming in the
'files' map from the client will have precedence, if the same
is also present in swift.
Change-Id: Ifa21fbcb41fcb77827997cce2d5e9266ba849b17
Story: #1755453
Task: 17353