On SLES12 use a systemd unit file to manage crowbar_join

This helps us blocking the chef-client daemon from starting before crowbar_join
is finished.
Cleanup some unneeded only_if/nof_if guards

The chef resources already to the right thing without them
Use the service resource for handing the crowbar_join service

It's a lot cleaner the calling chkconfig directly and we can move it outside
the if clause to share the code with SLE12.
Merge pull request #342 from rhafer/crowbar_join-systemd

On SLES12 use a systemd unit file to manage crowbar_join