doc: use newer configuration boilerplate for kojira
The Server Howto documentation describes how to configure Kerberos
authentication in kojira.conf.
Prior to this change, the Server Howto documentation's boilerplate
configuration copied the older stale format.
eea730300a5e6b40bf8c041a71b3f3c567de4abe added a dedicated section for
Kerberos authentication to kojira.conf. Copy this newer format into the
Server Howto documentation.
This also updates our Server Howto document to use the default
/etc/kojira/kojira.keytab file path, which we added in
f232e491941e888e4713a815148a00736445e7d4.