provide reasoning for out-of-date sles in code

Author: marcusdots

lib/vanagon/platform/defaults/sles-15-x86_64.rb

@@ -22,6 +22,9 @@
   plat.install_build_dependencies_with "zypper -n --no-gpg-checks install -y"
   plat.vmpooler_template "sles-15-x86_64"
   plat.docker_registry "registry.suse.com/suse"
+  # sles15.5 -> sles15.6 updates glibc in a breaking release.
+  # If built with 15.6 glibc, the openvox-agent on 15.5 will be broken.
+  # We bet on glibc backward compability and use the older sles/glibc to build.
   plat.docker_image "sle15:15.5"
   plat.docker_arch "linux/amd64"
 end