core: Calculate disk alias on add disk prior to setting the job message

The disk alias to be created is calculated during the execution phase so
if the user hasn't specified one, during the job creation the alias will
not appear in the task description, the setting of the alias (whether it's
default or the one the user supplied) is now done in the job message
calculation phase

Change-Id: Idee55da516bca986993e973dffc9058187e12a34
Signed-off-by: Tal Nisan <tnisan@redhat.com>
Bug-Url: https://bugzilla.redhat.com/1143967