Remove HEAT stack name randomization

This was enabled when tried to run functional tests for that stack
in only one tenant. Unfortunatly it leads to really difficult
error management.

This patch freeze the stack name and re-allow the functional test
script to clean the tenant if stack/images exist with that
same name.

Change-Id: I7ea157c5dcf2ba5d06eea94002c4a71e7edfdb38