install-chef-suse: Do not change bc-template-provisioner.json abusively

If we can avoid changing bc-template-provisioner.json for no reason when
trying to add existing ssh keys, then do not change it.
install-chef-suse: Rework error message when root password is not set

This is not fatal anymore if there are ssh authorized keys.
install-chef-suse: Check bc-template-provisioner.json exists

Before changing it, check that bc-template-provisioner.json exists. We
already do this for bc-template-dns.json.
Merge pull request #1834 from vuntz/install-chef-suse

Small fixes after merging the pull request about keeping ssh keys