Use a HeatIdentifier instance to create Parameters

This is the first commit in order to implement native pseudo parameters
for HOT templates.

Now to create a Parameters instance, a HeatIdentifier is used instead
of two variables (stack_name and stack_id)

Change-Id: Ifc914f8f6c943da73da5984452d424cf180b91e7
Implements: blueprint native-pseudo-params
Merge "Use a HeatIdentifier instance to create Parameters"