install-chef-suse: Update bc-template-provisioner.json only when needed

We should only run bc-provisioner-json.rb if /root/.ssh/authorized_keys
exists. Otherwise, even if it doesn't exist, it will output a json file
which will be different than the original file.

Thanks to Bernhard M. Wiedemann for spotting this!
Merge pull request #1836 from vuntz/install-chef-suse

install-chef-suse: Update bc-template-provisioner.json only when needed