cleanup(set_lsb_release): Rework release config setup

- Remove custom COREOS_* attributes from /etc/lsb-release
- Move dev image logic to dev_image_util
  For extra fun fix detection of local host URL for devserver.
- Remove weirdly verbose "DESCRIPTION" format.
- Add COREOS_RELEASE_BOARD back to /usr/share/coreos/release
  This is mostly just so update_engine and gmerge report the correct
  board name to devserver, informative-only on prod images.
- Remove version info from /etc/gentoo-release
- Switch from 'track' to 'group' terminology.