Rename PipelineArtifacts to StemcellArtifacts

It only knows about stemcell archives and is not specific to the pipeline bucket
Encapsulate knowledge about how promoting is done

Signed-off-by: Dmitriy Kalinin <dmitriy@pivotallabs.com>
Allows artifacts for gems, releases & stemcell to vary independantly

Signed-off-by: Dmitriy Kalinin <dmitriy@pivotallabs.com>