Configure generated child pipelines arbitrarily

[mahendrapaipuri]

Hello,

Currently, the child pipelines generation of ReFrame has limited or no configurability.

For example, the generated pipelines do not have image keyword which result in failed pipelines when any image other than default image of the runner is used (already submitted in 2134). There is also potential use cases where we need to include before_script and after_script directives in the pipelines. Sometimes we need to source ~/.bashrc file to make things conda, module load to work.

Coming to artefacts, ReFrame as of now provides only run_report of each job as an artefact. There is no way to configure or to tell ReFrame to get more artefacts like perflogs, stage, output, etc. It would be useful if the user can configure this in the general reframe_config.py and/or CLI interface. Similarly, in the current GitLab runner default configurations, artefacts are kept for 30 days and as we keep running more and more pipelines, the repository will get populated with these artefacts quite quickly and can reach the maximum allowable limit.