BE: Become a regular user after initialization
    
    Some parts of initialization (Kerberos ticket renewal, checking the
    keytab for the right principal) still require the root privileges. Drop
    privileges after initializing the back ends.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2370
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>