Refactor 'check_pipeline_status' and add 'trigger_pipeline'

Rename 'check_pipeline_status' to 'mirror_pipeline_result' in order to
better reflect what the function does.

Move determination of the pipeline out of this function to a new
function 'get_pipelines' so that this can be tested separately and we
can do different things depending on whether a pipeline exists or not.

Add function 'trigger_pipeline' to trigger a pipeline on a branch.

BCS 746-000635