Add ci option to skip automatic inclusion of the stack ppa

Some projects need a well controlled environment and need to have complete control over the ppas in use. Provide a ci option to skip inclusion of the stack "ppa:" value. For example:

stack:
  name: unity8
  ppa: ubuntu-unity/daily-build-next
  ...
  projects:
    unity8:
      use_stack_ppa: False
      ...

By default, "use_stack_ppa" would be True. True will trigger the current behavior.