Git push trigger workflow

Added trigger_workflow code to do the following:
  - Using trigger_id get the assemby and plan objects.
  - Call the build service for building the image

Added functional test for TriggerController.
Added unit test for testing the normal operation of trigger_workflow.

Note: Unit test for trigger_workflow failure case is not needed.
The only case where trigger_workflow will not call the _build_artifact()
method is when the plan file has no artifacts in it.
However, this scenario should be tested by assembly create unit tests.
The trigger_workflow will be called only after assembly create is successful.

Change-Id: Ie516306fc1355e2debca8b157c87dca3419956aa