fix(core_roller_upload): Use new payload names and upload path.

As of 4e85b172 the build system generates the update payload using the
coreos_production_update prefix, so use that instead. Use the release
version in the storage path instead of a disk image MD5; the version is
sufficiently unique and the no need for the obfuscation.

Drop the disk image copy in the download storage that I tacked in, that
will go into a new core_promote script that I have written but not
tested yet (need an alpha release to promote!!).
feat(set_lsb_release): Write configured group to /etc instead of /usr

The /usr images must be identical across groups so that information is
static. Eventually the default will be 'stable'
Merge pull request #243 from marineam/groups

More release/build script updates for beta prep