MDBF-1067: sles - change 15.7 name

As we haven't done a 15 SP7 release the
distinct name from the 15 (SP6) release enables
it not to be considered a minor upgrade and pass ci.

As our previous SP6 release was under the 15
name we keep that as is.

PR#786, is a better but not complete version of what
can happen after a release.

Author: grooverdan

os_info.yaml

@@ -205,7 +205,7 @@ sles-1507:
   image_tag: sles1507
   tags:
     - release_packages
-  version_name: 15
+  version_name: 15.7
   arch:
     - amd64
     - s390x