Move latest stemcell detection into stemcel environment
Get infrastructure from stemcell to avoid dependency on environment
StemcellBuilder returns path to new stemcell (better encapsulation)
Decouple StemcellPublisher from StemcellEnvironment
Use stemcell path provide by builder

* Allowed removal of stemcell filename guesswork in StemcellEnvironment
* StemcellEnvironment no longer acts as a middleman for publishing
Rename, stemcell_path is more accurate