DIB_IMAGE_ROOT_FS_UUID removed but still referenced in iso element and docs

As part of [1], the variable DIB_IMAGE_ROOT_FS_UUID was removed. However, this variable is still referenced within the DIB codebase:

$ grep -rn DIB_IMAGE_ROOT_FS_UUID
doc/source/developer/developing_elements.rst:257:``DIB_IMAGE_ROOT_FS_UUID``
diskimage_builder/elements/iso/cleanup.d/100-build-iso:164: EMITTED_KERNEL_CMDLINE_ARGS="root=UUID=$DIB_IMAGE_ROOT_FS_UUID"

Also, the variable is currently referenced by the fedora-atomic element[2] owned by the magnum project and its removal causes the fedora-atomic element to fail to build.

[1] https://git.openstack.org/cgit/openstack/diskimage-builder/commit/?id=e4e23897a13a3f3b9d28cc8d288990ab0fcc5b92
[2] https://github.com/openstack/magnum/blob/907415c984e95e09a8b872f3c551ae6108c91de1/magnum/drivers/common/image/fedora-atomic/finalise.d/80-fedora-atomic#L30