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.
> The same. Why do we have these redundant tests everywhere? Plus the grep is not needed here. chkconfig will return a non-zero status if the service is off.

Actually, I wonder why it doesn't use the ```service``` provider here instead of calling chkconfig. I updated that with the latest commit.