Enable 'pipeliner' tasks to be forced to run even if preceeding tasks have failed

[pjbriggs]

Adds new functionality to the pipeliner module, to enable tasks to be flagged as always_run when being added to a pipeline. If the pipeline fails in "deferred" failure mode then tasks that are flagged in this way will not be removed by the clean up operations even if they are dependent either directly or indirectly on the failed tasks.

For example for a simple notional pipeline:

ppl.add_task(first_task)
ppl.add_task(second_task,requires=(first_task,))
ppl.add_task(final_task,requires=(second_task,),always_run=True)

In this case if either of first_task or second_task fails then final_task will still be executed by the pipeline.

Note that any failed tasks will result in the pipeline being marked as "failed" (i.e. returning a non-zero exit code).

The expected application for this is reporting within the QC pipeline, where it would be useful to produce a report even if the pipeline has failed.