MDBF-1067: sles - change 15.7 name

[grooverdan]

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.

[RazvanLiviuVarzaru]

157 not 15.7
as in: sles157-amd64, sles157-s390x at the mirror

To clarify:

 mirror_url="https://rpm.mariadb.org/$branch/$arch"
  archive_url="https://archive.mariadb.org/mariadb-$branch/yum/$arch"
  if wget -q --method=HEAD "$mirror_url"; then
    baseurl="$mirror_url"
  elif wget -q --method=HEAD "$archive_url"; then
    baseurl="$archive_url"
  else

where $arch is:
os_name + str(OS_INFO[os_info_name]["version_name"]) + "-" + platform

opensuse example https://mirror.mariadb.org/yum/10.11/opensuse156-amd64/

[grooverdan]

yep, wasn't paying a great attention to the name as was intending on removing per #786