Add the pipeline API and db object

Migration of the trigger, image builder and heat stack
will be done in later patches.

Just to clarify what the meaning of pipeline is within this patch:
- A pipeline is the association between a plan, a mistral workbook
  and a git trigger. Together they form a working developement "pipeline".

A Pipeline is a running entity not a definition or DSL.
It's more like a heat stack or a mistral execution.

Change-Id: I921662e2d9ae95e72b3cd8622ddff3d5a0235b45
Merge "Add the pipeline API and db object"