Add support for configuring mod_auth_kerb.

Two new configuration variables are recognized:

 * CONF_BROKER_KRB_SERVICE_NAME for the KrbServiceName
 * CONF_BROKER_KRB_AUTH_REALMS for the KrbAuthRealms

If both are specified, mod_auth_kerb package gets installed
and enabled and AuthType Kerberos is configured with these
values, instead of the default AuthType Basic.