Update the Tuskar storage spec to reference Heat

Given the progress being made in Heat the most favourable option
now is to use it for parameter storage rather than Barbican.
This update highlights that slight change in direction in the
spec and moves the requirement of Barbican to be a possible
alternative.

Change-Id: I782277e256a3f179620ea58dc7da74ae4a62727e
Merge "Update the Tuskar storage spec to reference Heat"