backup,restore: Don't overwrite /etc/{passwd,group}
The /etc/passwd and /etc/group files are not saved and restored.
The DS user is always created on restore, and the PKI user is created
if a CA is being restored.
https://fedorahosted.org/freeipa/ticket/3866
Reviewed-By: Tomas Babej <tbabej@redhat.com>