fedora-cert: Fix username in kinit command example
    
    When running fedora-cert --configure-krb the code uses the username
    variable to write the ~/.fedora.upn file but uses opts.username in the
    output.
    
    The username may not have been explicitly passed as an option, in which
    case opts.username is False.  The username is derived from an existing
    or newly generated certificate if not provided.
    
    This causes potentially misleading output for the suggested kinit
    command:
    
        [tmz@localhost ~]$ fedora-cert --configure-krb
        Kerberos username configured. Run kinit False@FEDORAPROJECT.ORG
    
        [tmz@localhost ~]$ cat ~/.fedora.upn
        tmz
    
    Signed-off-by: Todd Zullinger <tmz@pobox.com>