If the configuration does not specify version for images or live media,
Pungi will create a default value based on `release_version`. If label
is used for the compose, the milestone from it will be appended to the
version (unless it's RC).
This change is backwards compatible: nothing changes when version is set
in configuration. If the version was missing before, building the
artifacts would fail. With this patch, default values will be supplied.
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>