Expand version field during image_build using version_generator
We can specify !VERSION_FROM_VERSION in version field during
image_build to expand it to correct release number.
Also implemented !RELEASE_FROM_DATE_RESPIN to provide correct
release number. This helps to keep Atomic Host media files name
produced by image_build during bodhi updates compose run
consistent with nightly run.
Related issue - https://pagure.io/pungi/issue/987
Signed-off-by: Sinny Kumari <sinny@redhat.com>