fix(*): Migrate image storage to new buckets

- Automated builds drop SDK and binary packages into
  gs://builds.developer.core-os.net/ and the new download URL is
  http://builds.developer.core-os.net/ (COREOS_DEV_BUILDS)
- Change default upload path to gs://users.developer.core-os.net/ for
  misc developer builds. Official builds go elsewhere and will just be
  configured in buildbot/jenkins so some COREOS_OFFICIAL stuff is gone.
- Automated builds of images go to a private bucket,
  gs://builds.release.core-os.net which later gets copied to
  gs://alpha.release.core-os.net and friends by core_promote.
fix(release_util): Add options to configure download path.

Since the new bucket scheme uploads images to a private staging area
first we need to configure the final location to generate vagrant json
metadata correctly.
Merge pull request #255 from marineam/buckets

fix(*): Migrate image storage to new buckets