Do not require a pipeline-job.sh for each pipeline step when using snappy kickoff

[xiamaz]

Is your feature request related to a problem? Please describe.
Currently snappy kickoff requires a pipeline-job.sh in each workflow step. This is mostly duplicated code and can be made much easier.

Describe the solution you'd like
Use a sensible default (either pipeline-job.sh in the .snappy-pipeline project directory or the library template), but allow for manually providing a pipeline-job.sh in each step for edge-cases.

Additional context

snappy-pipeline itself seems to be agnostic to the structure and location of pipeline-job.sh but is to some degree interlinked through the templating and creation of scripts when creating a new pipeline.