Implement an identifier stack_path()

This returns stack_name/stack_id, which is the form that needs to be
used with heatclient to avoid doing an extra stack lookup API call.

See the next change in this series to see this in use.

Change-Id: I41dcb732d36d702b7583b5e877fd074f86445a03
Related-Bug: #1291097
Provide the necessary inputs to enable HEAT_SIGNAL

To allow signalling via a native heatclient call using stack domain credentials
the following inputs are needed:
- the user ID, required to successfully auth with stack domain users, since it avoids
the need to have knowledge of the domain ID.
- the V3 endpoint as the auth_url, since it is domain aware
- a stack ID which consists of stack_name/stack_id, to avoid the need for an extra
  stack identifier lookup

Closes-Bug: #1291097
Change-Id: I677d1afb9bc40f84d9a71f376869946022a6635f
Merge "Implement an identifier stack_path()"
Merge "Provide the necessary inputs to enable HEAT_SIGNAL"