Idea: re-usable steps: pipeline steps as a function

[Dentrax]

Similar to melange/pipeline but for imagetest. So that we can use predefined pipelines like: test/output-check, test/port-check, etc.

A quote from Josh:

I go back and forth on this. I think it definitely makes sense to have reusable features, but reusable steps can be a huge foot gun given all the potential bash/hcl complexities, but it makes sense in some areas (liek the current helm install step), so my take is go for it and try it out, its easy enough to walk back