cleanup stemcell spec

 * add assets for non-AWS and aws + light contexts
Bosh::Dev:Stemcell knows its AMI ID
rename Bosh::Dev:Stemcell #is_light? to #light?
Bosh::Dev:Stemcell looks for AMI ID in the correct location

 * add DEFAULT_AWS_AMI_REGION
Bosh::Dev::Build can promote assets from pipeline
add .rspec file
stop "puts"ing in specs
ci:publish_artifacts uses Bosh::Dev::Build#promote_artifacts