Refactor for new cross-stack reference

[Kotaimen]

• [x] Process of convert config to boto3 call
• [x] Abstract stack Command objects
• [x] Separate boto3 call from click call

[Kotaimen]

Hi @GlieseRay,

Does that make sense to you to change output reference syntax from:

${Stage.Stack.OutputName}

To:

${Stage.Stack.Outputs.OutputName}

So we can add other stack references other than outputs in the future? This syntax is also identical with what nested stack uses:

Stack.Outputs.OutputName

[Kotaimen]

Hi @GlieseRay:

1. I'd still like to support following use case:

   ${Stage.Stack.Config.ConfigName}
   ${Stage.Stack.Parameters.ParameterName}
   ${Stage.Stack.Outputs.OutputName}
   

   This leaves us room for future expansion. of course with 2.1 released we need to keep support for old format:

   ${Stage.Stack.OutputName}
   

2. I think current "string template replacement" way of implement stack reference is not ideal and I would like to see it refactored.

3. When retrieving outputs from a referenced stack data is not cached, I'm wondering that could be improved?

[GlieseRay]

@Kotaimen I am going to use mustache or something else. Testing.