Use cross-platform /dev/null file

Issue raised in this Stack Overflow question:
http://stackoverflow.com/questions/17602148/how-to-deploy-cloudfoundry-to-aws

Answer from here:
http://stackoverflow.com/a/8682009/126042
Use full path, working dir is not the project root on CI
Remove missed cd, also datadog formatter needs an absolute path
Print pwd to help debug CI failures
DataDog formatter expects to be relative to load path
absolute path to bat shouldn't be needed, confirmed that we're in the correct dir
Single quote uninterpolated string per rubocop
Fix ci by chdiring into the bat dir before running bat specs
Encapsulate pipline's gems_dir_url
Convert quotes for uninterpolated strings per our styleguide
Remove unused BOSH_CI_PIPELINE_BUCKET env var
Make duplicate more obvious
DRY pipeline fog_storage
Avoid conflating real value with fake
Encapsulate 1 reason to get to fog_storage
Conceal knowledge of Fog in Pipeline
Encapsulate pipeline#s3_url
Cleaned up bulk uploader spec
Introduce build_id to Bosh::Dev::Pipeline, reducing artifact collision
rename spec to be same as implementation