Change VERSION for migration group specific flavor

[coolgw]

For 12sp5 project, we introduce new migration automation scenario 12sp5->15sp1, so some specific change will happen.
We create new flavor Migration-from-SLE12-SP5-to-SLE15-SPx for this kind of test cases, and triggered by VERSION=12-SP5 FlAVOR=Migration-from-SLE12-SP5-to-SLE15-SPx together with other 12sp5 migration test group. And we will have 12sp5 group view on openqa UI once NB is trigger
BUT we need change VERSION to 15-SP1 since only this can let migration code handle correctly to upgrade to 15SP1 in stead of 12-SP5.
This flavor will only using by migration group and will not impact any other group, once 12sp5 project finish, we can delete this special setting.
Welcome any comments.

• Related ticket: https://progress.opensuse.org/issues/52406
• Needles: n/a
• Verification run: n/a

[asmorodskyi]

I also not like what this patch is doing , but take into account that it influences only specific flavor used by patch's author and the fact that actually I can not suggests anything better because have low expertise in the subject I will merge it.
@okurz same logic I am applying to your comment as well ...