crowbar_register: Do not keep sledgehammer-specific configuration

When we first run chef-client, we need the FQDN to be resolvable, which
means we need it in /etc/hosts. But that's not something we want to
keep.

We also do not want to have "hostname" return the FQDN name anymore
after we're setup.
Merge pull request #303 from vuntz/crowbar_register-fix

crowbar_register: Do not keep sledgehammer-specific configuration