At the moment the output_resource module unconditionally adds a
copy step for every output resource in the taskrun.
However not every output resource will generate content to be
copied. Limit the copy to Storage and Git outoputs for now.

This is meant as a hotfix for bug tektoncd#401 until an implementation
of the image output resource is available via tektoncd#216. Until
image resource output is implemented, any consumer won't
have guarantee that the image has been actually built and pushed,
and won't know the hash of the created image.

Add a map that defines which output resource produces an output
that should be copied to the PVC. Add unit tests for it too.