When building a fedora atomic image using magnum's DIB element,
setting the DIB_RELEASE environment variable does not affect the
release of the fedora-atomic image built, even when
FEDORA_ATOMIC_TREE_REF and FEDORA_ATOMIC_TREE_URL take their default
values - fedora atomic 24 is always built.
Further, fedora atomic 24 is not supported by magnum for kubernetes
in newton onwards since the configuration of flannel was changed[1].
This change uses the DIB_RELEASE variable to set the default values
of FEDORA_ATOMIC_TREE_URL and FEDORA_ATOMIC_TREE_REF. This ensures
that fedora atomic images have an expected release, and one that by
default is supported by magnum (currently DIB_RELEASE defaults to 25
in diskimage-builder).
[1] https://bugs.launchpad.net/magnum/+bug/1679301
Change-Id: I9e1c846657ac9e65e2cdd8d32c96cdbf85900ba0
Closes-Bug: #1699766
Closes-Bug: #1699765