crowbar_register: Make the check for SLE12 more future proof

This is basically the same as f96652697, just for crowbar_register
Fix crowbar_join installation on SLE12 nodes

The crowbar_join.sh as installed on the client nodes was still using
wrong sntp command line on SLE12 because we didn't pass the template the
target_platform_version parameter.
Merge pull request #341 from rhafer/sle12

crowbar_join and crowbar_register fixes for SLE12